Final Mineral Descriptions

MINERAL DESCRIPTIONS

ACTINOLITE:

Composition: Ca2(Mg,Fe)5Si8O22(OH)2; a double-chain inosilicate; an amphibole group mineral;

forms a solid solution series with tremolite in which < 10% iron (Fe+2

) substitutes for magnesium (Mg+2

);

iron-rich varieties of actinolite (>50% Fe+2

substituting for Mg+2

) are called ferro-actinolite

Crystal System: Monoclinic (2/m): a = 9.87Ǻ, b = 18.12Ǻ, c = 5.30Ǻ; α = 90o, β = 105

o, γ = 90

o

Crystal Habit: slender, bladed prismatic to acicular with diamond-shaped cross-sections;

crudely radiated to foliated to massive aggregates

Macroscopic Properties:

Hardness: 5-6

Specific Gravity: 3.2 – 3.3

Cleavage/Fracture two orientations of prismatic cleavage {110} not at right angles (56o

and 124o); very good; splintery

Diaphaneity: somewhat translucent

Colors: various shades of medium to dark green; darker for iron-rich varieties

Streak: white

Luster: vitreous

Other: commonly alters to chlorite, talc and carbonate

Diagnostic Properties: the hardness and cleavage are distinctive of amphibole;

actinolite possesses a darker green color than most tremolite and a lighter green color

than most hornblende; ferro-actinolite and hornblende can be difficult to distinguish,

although slender prismatic-acicular crystals and a radiated habit is suggest ferro-actinolite

Microscopic Properties:

Refractive Indices: nα = 1.613-1.628; nβ = 1.627-1.644; nγ = 1.638-1.656; increasing

with increasing iron content

Relief: moderate to moderate-high; positive

Color/Pleochroism: colorless to pale green: pleochroism increases with iron content,

also pale brown, pale yellow, yellow-green, green, dark green and bluish green

Birefringence: moderate (0.024-0.028); lower to upper second order colors

Twinning: simple paired twins are common

Optic Sign: Biaxial (–); 2V = 73-88o; high to very high 2V; decreases with increasing

iron (Fe)

Other: Cross-sections display symmetrical extinction; longitudinal sections are length

slow; maximum extinction angles are 12-17o

Diagnostic Properties: the diamond-shaped cross-section and two cleavages at 56o and

124o distinguish actinolite as an amphibole; tremolite is colorless and has lower refractive

indices; anthophyllite has lower birefringence and is orthorhombic, so has parallel

extinction in longitudinal sections; cummingtonite has higher refractive indices, paler,

brown colors and is less pleochroic; actinolite can be difficult to distinguish from

hornblende, but usually has smaller extinction angles

Occurrences/Associations: Actinolite occurs primarily in low-grade mafic and, less commonly

calcareous, metamorphic rocks such as schist, skarn and calcsilicate rocks formed in the

greenschist and, less commonly albite-epidote hornfels, facies where it is associated with

minerals such as calcite, dolomite, albite, epidote, chlorite, anthophyllite and talc. It also occurs

in rocks of the blueschist facies where it is associated with glaucophane, pumpellyite and

lawsonite.

Uses: compact aggregates, called nephrite, are used as an ornamental stone and in jewelry

AEGERINE: formerly also called ACMITE

Composition: NaFeSi2O6; forms a solid solution series with calcic clinopyroxenes such as

augite in which significant amounts of calcium (Ca+2

) substitute for sodium (Na+1

) and aluminum

(Al+3

), ferrous iron (Fe+2

) and/or magnesium (Mg+2

) substitute for ferric iron (Fe+3

); intermediate

varieties with between 20 - 80% ferric iron (Fe+3

) are called aegerine-augite

Crystal System: Monoclinic (2/m): a = 9.7, b = 8.8, c = 5.3; α = 90o, β = 105-l08

o, γ = 90

o

Crystal Habit: occurs as stubby prismatic crystals with 4- to 8-sided cross-sections or as slender

prismatic to acicular crystals; occurs divergent aggregates or in fibrous aggregates or as

disseminated crystals


Hardness: 6 – 6½; hard

Specific Gravity: 3.4 – 3.6; moderate to moderate plus

Cleavage/Fracture: two cleavages {110}, near 90o (87

o and 93

o); good only; prominent

parting may bisect the cleavages

Diaphaneity: somewhat translucent to barely translucent

Colors: typically various shades of dark green to black; also yellow green, brown or

reddish brown

Streak: white to pale green-gray

Luster: vitreous

Other: commonly alters to uralitic chlorite and uralitic amphibole

Diagnostic Properties: distinguished from amphiboles by the crude, right angle cleavage

and in crystals by the squarish 4- 8-sided cross-sections; cannot be distinguished from

other calcic clinopyroxenes such as augite in hand sample


Refractive Indices: nα = 1.751 – 1.776; nβ = 1.785 – 1.820; nγ = 1.801 – 1.836; aegerine-

augite has lower indices

Relief: high, positive

Color/Pleochroism: distinctly pleochroic in pale to dark green, bright emerald green to

blue-green, olive green, yellow-green, yellow-brown and honey-yellow brown; aegerine-

augite possesses paler colors, mostly in yellow and green hues

Birefringence: 0.040 – 0.060; high; upper second to fourth order colors are somewhat

obscured by the strong colors of the mineral; aegerine-augite possess lower, moderate to

high birefringence with upper first to upper second order colors

Twinning: simple paired twins and lamellar twins occur

Optic Sign: Biaxial (–), 2Vx = 60-90o; high to very high 2V; aegerine-augite is

generally (+) with a large 2V = 70-90o

Other: cross-sections (showing near right angle cleavage) display symmetrical

extinction; longitudinal sections are length fast; maximum extinction angles are 2-20o;

aegerine-augite is 15-38o

Diagnostic Properties: distinguished from green amphiboles by its 2 orientations of

cleavage near right angles and squarish 4- 8-sided cross sections; green amphiboles are

also length slow and possess larger extinction angles; the darker colors, strong

pleochroism and (–) optic sign distinguish aegerine from augite and the transitional

mineral aegerine-augite which also possess larger extinction angles, lower refractive

indices and lower birefringence

Occurrences/Associations: Aegerine and aegerine-augite typically occur in alkali-rich igneous

rocks, both silica oversaturated rocks such as alkali felsdspar granite, approximately saturated

rocks such as syenite and silica undersaturated rocks such as foid syenite. They are commonly

associated with potassic feldspars, sodic amphiboles and either quartz or felspathoids. Aegerine

also occurs in some high P/T Franciscan trajectory blueschist facies metamorphic rocks formed

in subduction zones, where it is associated with glaucophane, riebeckite, garnet and albite-rich

plagioclase.

Uses: Neither aegerine nor aegerine-augite have commercial value.

AGATE: see CHALCEDONY

ALBITE: see PLAGIOCLASE

ALMANDINE(ALMANDITE): see GARNET

ANALCIME: (ANALCITE)

Composition: NaAlSi2O6•H2O; a tectosilicate mineral; a zeolite mineral; some substitution of

potassium (K+1

) and sodium (Ca+2

) for calcium (Na+1

) commonly occurs; the latter is balanced

by substitution of aluminum (Al+3

) silicon (Si+4

)

Crystal System: Isometric (4/mbar32/m): a1 = 13.72Å, a2 = 13.72Å, a3 = 13.72Å; α = 90o, β =

90o, γ = 90

o

Crystal Habit: occurs as equant, trapezohedral crystals that resemble soccer balls; may possess

cubic modifying faces; often as granular aggregates of such crystals or as more finely-crystalline

massive aggregates or disseminated crystals



Specific Gravity: 2.2 – 2.3; low

Cleavage/Fracture: generally not discernable

Diaphaneity: transparent to somewhat translucent

Colors: typically colorless to white; also shades of gray, green, yellow and pink

Streak: white

Luster: vitreous

Diagnostic Properties: The white color, hardness and trapezohedral crystals that

resemble soccer balls are diagnostic; finely crystalline aggregates generally require XRD

analysis for accurate identification


Refractive Indices: n = 1.479 – 1.493

Relief: moderate, negative

Color/Pleochroism: colorless

Birefringence: none, since isotropic

Twinning: none

Optic Sign: none, since isotropic

Diagnostic Properties: the rounded cross-section of euhedral trapezohedral crystals and

the negative relief are useful; leucite has similar crystals, but much lower refractive

indices and higher negative relief; sodalite possesses similar refractive indices and

dodecahedral crystals; other zeolites are anisotropic; finely crystalline analcime requires

XRD or other analytical methods for proper identification

Occurrences/Associations: Analcime is a relatively widespread zeolite mineral. Coarse crystals

are especially common in cavities, including vesicles, in mafic/basic rocks such as basalt and,

less commonly, intermediate volcanic rocks such as andesite. It also occurs as phenocrysts in

foid-bearing basalts. Analcime is a common mineral in very low-grade regional metamorphic

rocks of the zeolite facies, typically associated with heulandite and also occurs in some contact

metamorphic aureoles. Analcime is also reported from alkaline soils formed in desert

environments, as an authigenic mineral in sedimentary rocks and as an alteration product of tuff.

Uses: Analcime, like most natural and synthetic zeolites, is used, after heating and dehydration,

to produce materials that remove water vapor from carbon dioxide, from refrigerants such as

Freon and from many organic chemicals used in industrial applications. It is also used as a

molecular sieve used to separate molecules of different sizes as in separating nitrogen from air to

produce nearly pure oxygen gas. Because zeolites can selectively absorb selected molecules,

they are widely used in applications that include reducing the hardness of water for cleaner

laundry by the removal of calcium, removing heavy metals from mine waters and industrial

waste and radioactive isotopes from nuclear waste, in the treatment of wastewater by the removal

of undesirable dissolved species such as ammonia and in the cleaning up of oil spills.

ANDESINE: see PLAGIOCLASE

ANDALUSITE:

Composition: AlAlOSiO4; a nesosilicate (orthosilicate) mineral, polymorphic with kyanite and

sillimanite; small amounts of iron (Fe+3

) or manganese (Mn+3

) may substitute for aluminum

(Al+3

)

Crystal System: Orthorhomibc (2/m2/m2/m): a = 7.79Å, b = 7.91Å, c = 5.56Å; α = 90o, β =

90o, γ = 90

o

Crystal Habit: typically in prismatic to stubby prismatic crystals with roughly square cross-

sections; mostly as disseminated crystals; less commonly as granular aggregates


Hardness: 7 – 7½; very hard

Specific Gravity: 3.1 – 3.2; moderate

Cleavage/Fracture: two cleavages {110}, near 90o (89

o and 91

o); good only

Diaphaneity: somewhat to quite translucent; rarely transparent

Colors: most commonly reddish brown to reddish gray; also pinkish, violet, greenish

Streak: white

Luster: vitreous

Other: carbonaceous inclusions in cross-like or rectangular forms occur in the variety

called chiastolite; commonly alters to sericite or chlorite

Diagnostic Properties: the prismatic crystals with square cross-sections are

characteristic and the chiastolite inclusions, where present, diagnostic; the hardness and

reddish toned colors are also typical


Refractive Indices: nα = 1.629 – 1.640; nβ = 1.633 – 1.645; nγ = 1.638 – 1.650

Relief: moderately high, positive

Color/Pleochroism: colorless; less commonly, very slightly pleochroic; pale pink or red,

pale green or pale yellow

Birefringence: 0.009 – 0.013 low; maximum first order gray to yellow colors; higher for

manganese (Mn) rich varieties

Twinning: rarely observed

Optic Sign: Biaxial (-), 2Vx = 71-88o; high to very high 2V

Other: square cross-sections display symmetrical extinction and may display near right

angle cleavages parallel to faces; longitudinal sections are length fast, display parallel

extinction and one cleavage;

Diagnostic Properties: the combination of color, moderately high relief, nearly square

cross-sections with 2 sets of near right angle cleavage, low birefringence, length fast

character and parallel extinction, with biaxial (–) optics and large 2V are characteristic;

chiastolite inclusions of symmetrical carbonaceous graphite are diagnostic; sillimanite is

length slow, has higher birefringence, only one orientation of cleavage and has a typically

fibrous to “swirled” habit; topaz can possess similar color, but is optically (+) with a

smaller 2V, possesses 8-sided cross-sections, and only one basal cleavage, not visible in

basal cross-sections; hypersthene (see orthopyroxene) often possesses 8-sided cross-

sections and somewhat more color and pleochroism and has a different association, in

mafic-ultramafic igneous and metamorphic rocks; enstatite (see orthopyroxene) is

optically (+)

Occurrences/Associations: Andalusite is the low pressure polymorph of aluminum silicate and

typically forms in pelitic metamorphic rocks at temperatures below 700oC and pressures of less

than 4kbars and is associated with chlorite, chloritoid, biotite, muscovite, almandine garnet,

cordierite and staurolite. It is therefore abundant in rocks formed by contact metamorphism in

the albite-epidote hornfels and hornblende hornfels facies. It is also common in regional

metamorphic rocks such as schist and gneiss produced in the greenschist and amphibolite facies

along Buchan trajectories where T/P ratios are fairly high. Andalusite also occurs in

peraluminous igneous rocks including granitoids and pegmatite, although whether or not it is a

primary mineral in such rocks is controversial.

Uses: Andalusite is mined extensively to be refined for use as a refractory material used in the

manufacture of the porcelain used spark plugs and high alumina bricks for lining blast furnaces

and kilns. It may also be used as an abrasive.

ANDRADTITE: see GARNET

ANHYDRITE:

Composition: CaSO4; a sulfate mineral

Crystal System: Orthorhombic (2/m2/m2/m): a = 6.99Ǻ, b = 7.00Ǻ, c = 6.24Ǻ; α = 90o, β =

90o, γ = 90

o

Crystal Habit: individual crystals thick, tabular; less commonly stubby prismatic; most

commonly in granular or massive aggregates; sometimes acicular-capillary crystals in fibrous or

even radial aggregates


Hardness: 3 – 3½; moderate


Cleavage/Fracture: three cleavages at 90o; all pinacoidal {010} and {100} very good

and {010} good, so can resemble the cubic cleavage in many isometric minerals

Diaphaneity: generally quite translucent to somewhat translucent

Colors: commonly pale purple to white or gray; may be pale blue, red or brown

Streak: white

Luster: vitreous

Other: commonly alters, by hydration, to gypsum (CaSO4•2H2O) and forms by the

dehydration of gypsum

Diagnostic Properties: The three distinct orientations of cleavage at right angles and

lack of effervescence in HCl distinguish anhydrite from calcite and other rhombohedral

carbonates and it is harder than gypsum which is soft



Relief: moderate, positive


Birefringence: 0.042 – 0.045; so moderate-high, with maximum lower third-order

colors, but most sections display lower birefringence, so examine several crystals

Twinning: simple, paired twins occur, as do lamellar sets at right angles

Optic Sign: Biaxial (+), 2Vz = 36-45o; moderate

Other: extinction is parallel to cleavage traces

Diagnostic Properties: the right angle cleavage and high birefringence are characteristic;

gypsum has lower, negative relief, much lower birefringence, and a larger 2V

Occurrences/Associations: Most anhydrite occurs in marine evaporite sequences where it

precipitates from sea water that is undergoing significant desiccation by net evaporation in a

restricted sea in a warm, dry climate or by the dehydration of gypsum precipitated under similar

conditions. Anhydrite is an important constituent of the cap rocks of salt domes. In both the

preceding cases, anhydrite is associated with gypsum, halite and calcite and in salt domes with

native sulfur. Smaller amounts of anhydrite form by the hydrothermal alteration of carbonate

sedimentary rocks such as limestone and dolostone, in the oxidized portions of hydrothermal

sulfide vein and replacement deposits and in amygdules in volcanic rocks.

Uses: Ground up anhydrite is used as a soil conditioner; it is also used in cement to slow down

the setting time where that is desirable

ANTHOPHYLLITE:

Composition: (Mg,Fe)2(Mg,Fe)5Si8O22(OH)2; double-chain inosilicate; an amphibole group

mineral; forms a solid solution series with gedrite [(Mg,Fe)2(Mg,Fe)2Al2 Al2Si8O22(OH)2] in

which aluminum (Al+3

) substitutes both for magnesium (Mg+2

) and/or ferrous iron (Fe+2

) in

octahedral sites and for silica (Si4+

) in tetrahedral sites; related to cummingtonite, but larger

amounts of iron (Fe+2

) in cummingtonite cause it to crystallize in the monoclinic, rather than the

orthorhombic, system

Crystal System: Orthorhombic (2/m2/m2/m): a = 18.54Å, b = 18.03Å, c = 5.28Å; α = 90o, β =

90o, γ = 90

o

Crystal Habit: slender, bladed prismatic; crudely with diamond-shaped cross-sections; in

foliated-lamellar to massive aggregates; also as acicular crystals in fibrous to radiated aggregates


Hardness: 5½ - 6


Cleavage/Fracture: two orientations of prismatic cleavage {110}, not at right angles

(56o and 124

o); very good

Diaphaneity: somewhat to slightly translucent

Colors: typically clove brown; also gray-brown, brownish green and yellow brown to

white; color may vary within single aggregate

Streak: white

Luster: vitreous to silky; also subvitreous

Other: commonly alters to chlorite, talc and/or serpentine

Diagnostic Properties: Distinguished from most amphiboles by the clove brown (to gray

brown to brownish green) color and commonly; the fibrous to lamellar-foliated habit are

also distinctive; difficult to distinguish from cummingtonite which is usually a darker

brown; pyroxenes possess near right angle cleavage (when it is discernible)



Relief: moderate to high; positive

Color/Pleochroism: weakly pleochroic; pale tan, clove brown, greenish yellow, pale

gray brown, pale yellow brown, gray green, dark brown

Birefringence: 0.13 – 0.24; low to moderate; maximum colors from first order yellow to

second order reds and blues

Twinning: none

Optic Sign: Biaxial (+) or (–); mostly positive; 2Vx = 65-90o; 2Vz = 58 – 90

o; high to

very high 2V

Other: cross-sections display symmetrical extinction; displays extinction parallel to

cleavage traces in longitudinal sections

Diagnostic Properties: distinguished from pyroxenes by 2 sets of cleavage not at right

angles; cleavage may not be visible in fibrous aggregates; all monoclinic amphiboles

possess angular extinction; the pleochroism and pale colors in shades of tan, clove brown,

greenish-yellow, gray-green and brown is somewhat similar to cummingtonite, but

cummingtonite has angular extinction, as do all monoclinic amphiboles

Occurrences/Associations: Anthophyllite is common in medium- to high-grade mafic

metamorphic rocks such as schist and gneiss, where it is associated with cordierite, plagioclase,

and garnet. It also occurs in some magnesian (ultramafic) metamorphic rocks, often by the

retrograde metamorphism of olivine and pyroxene, where it is associated with serpentine and

talc.

Uses: Anthophyllite has been used as a substitute for chrysotile asbestos which was long used in

fireproofing and insulation. However, it turns out to have similar negative health implications.

ANTIGORITE: see SERPENTINE

APATITE:

Composition: Ca5(PO4)3(F,Cl,OH); but complex, with complete substitution of F-1

, Cl-1

and

OH-1

possible between three end members: fluorapatite, clorapatite and hydroxylapatite;

phosphate may be partially substituted for by sulfate, hydroxylcarbonate, chromate, or silicate

with sodium replacing some of the calcium to maintain electrical neutrality; the sedimentary

apatite called collophane is mostly chlorapatite and hydroxylcarbonate apatite

Crystal System: Hexagonal (6/m): a1 = 9.32 – 9.65Ǻ, a2 = 9.32 – 9.65Ǻ, c = 6.78 – 6.90; α =

120o, β = 90

o, γ = 120

o

Crystal Habit: typically in stubby to elongate hexagonal prisms or columns {11bar20} which

are commonly terminated by basal pinacoid {0001} and dipyramidal {10bar11} faces; often in

granular to massive aggregates; the cryptocrystalline, sedimentary apatite called collophane

occurs as ooids, colloform masses, encrustations and disseminated material in sedimentary rocks

called phosphorites


Hardness: 5; upper end of moderate


Cleavage/Fracture: generally not discernible, one poor basal cleavage; uneven to

conchoidal fracture

Diaphaneity: transparent to quite translucent

Colors: variable; typically shades of green to blue-green, but also yellow-green, brown,

purple, blue, pink and yellow-orange; collophane is typically brownish

Streak: white

Luster: vitreous to resinous

Diagnostic Properties: the hexagonal crystals, greenish color and hardness are

characteristic; beryl is harder and lacks dipyramidal faces


Refractive Indices: nω = 1.633 – 1.650; nε = 1.629 – 1.647; due to variable composition;

carbonate-rich varieties possess lower indices

Relief: moderate to moderately high; positive

Color/Pleochroism: colorless to very pale shades of green; not pleochroic; surface has a

distinctive “stippled” appearance in plane light; collophane is brownish

Birefringence: 0.003 – 0.005; very low; first order white to gray; collophane is nearly

isotropic; carbonate-rich apatite possesses somewhat higher birefringence (upper first

order colors)

Optic Sign: uniaxial (–)

Other: euhedral cross-sections are hexagonal; longitudinal sections are length fast

Diagnostic Properties: the moderate relief, very low birefringence, uniaxial (–) optics

and length fast character, lack of recognizable cleavage and, where present,

hexagonal cross-sections are diagnostic

Occurrences/Associations: Apatite is widely distributed in igneous, sedimentary and

metamorphic rocks. It is a common accessory mineral a wide variety of igneous rocks that range

from granitoids and alkali syenites through gabbroic rocks; large crystals occur in some granitic

pegmatites. Apatite is especially common in sedimentary rocks, especially in phosphorites

composed of collophane; it is the major component of bones and teeth which may directly or

indirectly contribute to such deposits where it is associated with carbonate rocks, siliceous rocks

and mudrocks. Apatite is also common in metamorphic rocks, especially marbles, skarns and

calcsilicate rocks; large crystals are quite common in skarns.

Uses: Apatite, especially in the form of collophane, is a major source of phosphate which is

widely used in agricultural fertilizers, especially now that deposits of guano have been largely

exhausted. It is also a source of fluoride used in drinking water and mouthwashes to help

prevent dental cavities. Fine quality, transparent apatite can provide gemstones, but it is too soft

to be of high value for this purpose.

ARAGONITE:

Composition: CaCO3; a carbonate; polymorphic with calcite; isostructural with witherite,

strontianite and cerrusite, all members of the orthorhombic (aragonite) group of carbonate

minerals; strontium (Sr+2

) commonly substitutes for some calcium (Ca+2

)


90o, γ = 90

o

Crystal Habit: acicular-prismatic or tabular-bladed crystals; divergent to radiating aggregates;

penetration twins commonly produce columnar pseudohexagonal crystals with striations parallel

to long axes; also in stalactitic and banded aggregates in cavestones


Hardness: 3½ - 4; moderate

Specific Gravity: 2.94; moderate

Cleavage/Fracture: one cleavage; pinacoidal {010} fair; another poor, so both difficult

to discern; subconchoidal fracture

Diaphaneity: translucent to transparent

Colors: colorless to white; impurities can produce other colors including yellow and

reddish tones

Streak: white

Luster: vitreous

Other: effervesces readily in cold, dilute hydrochloric acid (HCl); commonly inverts to

its polymorph calcite; may also be replaced by dolomite; so becomes rarer with age in the

geologic record

Diagnostic Properties: Resembles calcite in occurrence and ready effervescence in

HCl, but lacks rhombohedral cleavage; also has different crystal forms, is marginally

harder and possesses a somewhat higher specific gravity


Refractive Indices:

Relief: very low negative to moderate-high positive; most sections produce a

“twinkling” effect as the relief changes when stage is rotated under plane light


Birefringence: 0.155; very high-extreme; high order faded colors (“creamy white”)

Twinning: alternating penetration twins common; some polysynthetic twinning

Optic Sign: Biaxial (–); 2Vx = 18 - 19o (small 2V)

Other: cross-sections may be psuedohexagonal; longitudinal sections are length slow;

parallel extinction

Diagnostic Properties: distinguished from rhombohedral carbonates (e.g., calcite

and dolomite) by the lack of rhombohedral cleavage and by being biaxial (–), rather than

uniaxial (–); witherite is similar and difficult to distinguish without XRD, but it is a much

less common mineral and possesses a markedly higher specific gravity; strontianite has a

lower 2V, lower refractive indices and better cleavage

Occurrences/Associations: Aragonite is the high pressure polymorph of calcium carbonate

(CaCO3), which is reflected by its occurrence in high P/T metamorphic rocks of the blueschist

facies where it is associated with minerals that include lawsonite, pumpellyite, glaucophane,

muscovite, pyrophyllite and kyanite. Despite this, aragonite is also a common mineral in surface

environments where it is used by organisms to secrete shells and precipitated in caves and

around hot springs. It also precipitates from warm sea water as fine aragonite needles and as

ooids. Less common aragonite occurs with zeolites in amygdules in volcanic rocks and with

siderite in some sedimentary iron deposits.

Uses: Aragonite can be used in the same way as calcite, but because it is scarcer and deposits are

smaller, aragonite has limited commercial value except as a local source of lime for whitewash

ARSENOPYRITE:

Composition: FeAsS; a sulfarsenide mineral

Crystal System: Monoclinic (2/m): a = 9.57Ǻ, b = 5.68Ǻ, c = 6.43Ǻ; α = 90o, β = 90+

o, γ = 90

o;

nearly orthorhombic

Crystal Habit: slender prismatic crystals common; crudely foliated to massive aggregates


Hardness: 5½ - 6

Specific Gravity: 6.07; high to very high

Cleavage/Fracture: none easily discernible

Diaphaneity: opaque

Colors: silver white

Streak: black

Luster: metallic

Other: smells of garlic when crushed or powdered

Diagnostic Properties: the garlic smell when crushed is distinctive; the silver white

color distinguishes arsenopyrite from pyrite and marcasite which are pale brass yellow;

the prismatic habit with rhombic cross-sections help to distinguish it from cobaltite

Microscopic Properties: opaque; white in reflected light

Occurrences/Associations: Arsenopyrite occurs most commonly in high temperature

(hypothermal) vein and replacement deposits. It also occurs in contact metamorphic rocks such

as skarns and in a few pegmatites. Commonly associated minerals include cassiterite,

wolframite, scheelite, gold, pyrite, pyrrhotite, chalcopyrite, galena, and sphalerite.

Uses: Arsenopyrite is not an important economic mineral; it can be used as a source of arsenic,

but this is typically obtained from the smelting of other minerals. Arsenic is used in herbicides,

pesticides and insecticides, in some pharmaceuticals and with lead in the production of shot

metal.

AUGITE:

Composition: (Ca,Na)(Mg,Fe,Fe,Al,Ti)Si2O6; single-chain inosilicate; a clinopyroxene; shows

extensive solid solution with diopside (CaMgSi2O6) and hedenbergite (CaFeSi2O6) and, at

high temperatures, with pigeonite (Fe,Mg,Ca)2Si2O6

Crystal System: Monoclinic (2/m): a = 9.75Å, b = 8.90Å, c = 5.25Å; α = 90o, β = 106

o, γ = 90

o

Crystal Habit: typically as stubby prismatic {210} crystals with squarish four- to eight-sided

cross sections; commonly in granular aggregates or coarse cleavable aggregates or as



Hardness: 5½ - 6; hard


Cleavage/Fracture: two prismatic cleavages {210}, near right angles (87o and 93

o);

prominent parting in some examples

Diaphaneity: moderately translucent to barely translucent

Colors: typically medium to dark green to greenish black; also brownish black

Streak: pale gray-green

Luster: vitreous to subvitreous

Other: commonly alters to chlorite, biotite, limonite, hematite and clay minerals

Diagnostic Properties: augite cannot be distinguished in hand specimen from other

clinopyroxenes with similar colors; the crude, nearly right angle cleavage and/or four- to

eight-sided squarish crystals distinguish it from amphiboles with similar color


Refractive Indices: nα = 1.674 – 1.724; nβ = 1.686– 1.732; nγ = 1.708 – 1.752


Color/Pleochroism: colorless to rather pale shades of green, grayish green, brownish

green, blue green, yellow green and purple brown; very slightly pleochroic

Birefringence: 0.022 – 0.030; moderate; maximum second order green to second order

red colors

Twinning: simple paired and lamellar twins common

Optic Sign: Biaxial (+); 2Vz = 35 – 60o; moderately low to moderate 2V

Other: squarish, 4-8 cross-sections (showing near right angle cleavage) display

symmetrical extinction; longitudinal sections are length slow; maximum extinction

angles are 39 - 52o; aegerine-augite is 15-38

o; exsolution lamellae of orthopyroxene or

pigeonite are common; phenocrysts commonly zoned

Diagnostic Properties: the squarish 4-8 sided cross-sections and two orientations of

cleavage near right angles distinguish augite from most amphiboles; pigeonite is similar,

but has a lower 2V (< 32o) and often displays pinkish colors in addition to green and

brown; orthopyroxenes show parallel extinction in all longitudinal sections; olivine lacks

cleavage and color and has much higher birefringence and generally higher 2V; iron-rich

orthopyroxene (hypersthene) and iron-rich olivine are also optically (–)

Occurrences/Associations: Augite is an important rock-forming mineral. It is an essential

primary constituent of many mafic/basic and ultramafic/ultrabasic igneous rocks including

gabbro, basalt, peridotite (lherzolite and wehrlite) and pyroxenite (websterite and

clinopyroxenite). Mafic/basic rocks are important constituents of oceanic crust and of intrusive

rocks in continental crust, including large layered gabbroic intrusions (LLGI).

Ultramafic/ultrabasic rocks are the dominant rock type in the upper mantle. Augite, often iron-

rich also occurs as an accessory mineral in diorite, andesite, granodiorite and tonalite. Augite

also occurs in relatively high-grade mafic and magnesian metamorphic rocks such as hornfels,

amphibolite, gneiss and granulite produced in the pyroxene hornfels, upper amphibolite and

granulite facies. It is often the product of dehydration reactions that involve hornblende, as

metamorphic temperatures increase. Augite is not very chemically stable in weathering

environments, so is not an important constituent of detrital sediments.

Uses: Augite has no commercial value. It is a constituent of some dimension stone used for

countertops.

AZURITE:

Composition: Ca2(Mg,Fe)5Si8O22(OH)2; a hydrous carbonate;


o, γ = 90

o

Crystal Habit: short prismatic to tabular individual crystals; acicular crystals, commonly in

radiating, banded, colloform (botryoidal) aggregates; also as drusy encrustations


Hardness: 3½ -4

Specific Gravity: 3.77

Cleavage/Fracture: 2 orientations of prismatic cleavage not at 90o; perfect; a third

orientation of fair cleavage is generally not discernible; in finely crystalline aggregates

no cleavage is visible macroscopically

Diaphaneity: quite translucent to somewhat translucent

Colors: various shades of deep blue; often intense azure blue

Streak: lighter shades of blue


Other: effervesces in cold, dilute hydrochloric acid (HCl); commonly alters to malachite

Diagnostic Properties: The deep blue color, blue streak and effervescence in HCl are

diagnostic


Refractive Indices: nα= 1.730; nβ = 1.756; nγ = 1.836

Relief: high to very high positive

Color/Pleochroism: blue

Birefringence: 0.011; low

Optic Sign: biaxial (+); 2Vz = 67o (moderately large)

Diagnostic Properties: The blue color, high relief, low birefringence and biaxial (+)

optics with a large 2V are diagnostic; the association with other oxidized zone minerals is

helpful

Occurrences/Associations: Azurite most commonly occurs in the oxidized zone of

hydrothermal vein and replacement deposits that contain copper-bearing sulfide minerals such as

chalcopyrite and bornite that are oxidized by descending, acidic (CO2-rich) meteoric water. It is

typically associated with malachite, cuprite, limonite (goethite), smithsonite and cerussite,

among others.

Uses: a minor ore of copper; large masses are prized by collectors as decorative pieces; when

powdered azurite provides blue pigments

BARITE:

Composition: BaSO4; a sulfate mineral; isostructural with celestite; a complete solid solution

series is possible and Sr typically substitutes for Ba to some degree producing variable specific

gravity, refractive indices and birefringence


90o, γ = 90

o

Crystal Habit: individual crystals are typically tabular to bladed; less commonly prismatic; most

commonly divergent aggregates; in “barite rose” tabular crystals occur in roseiform aggregates;

also occurs in granular or massive aggregates; also in oolitic, pisolitic and concretionary

aggregates


Hardness: 2½ – 3½; moderate

Specific Gravity: 4.5; high

Cleavage/Fracture: four cleavage orientations; one pinacoidal {001} perfect cleavage is

at right angles to two prismatic {210} very good cleavages that are not at right angles to

each other (78o and 102

o); a fourth cleavage is pinacoidal {010} fair, so not readily

discerned


Colors: commonly colorless to white or gray; may be pale blue, green, red or brown

Streak: white

Luster: vitreous to pearly

Other: commonly alters to witherite; may be replaced by calcite, dolomite or quartz

Diagnostic Properties: Barite has an unusually high specific gravity for a non-metallic

mineral; the three cleavages in anhydrite are at 90o; celestite has a lower specific gravity

and commonly a more bluish color



Relief: moderate to moderately-high, positive


Birefringence: 0.010 – 0.013; so low-moderate with maximum first-order yellow colors,

but most sections display low birefringence

Twinning: usually not twinned, but lamellar twins do occur

Optic Sign: Biaxial (+), 2Vz = 36-40o; moderate

Other: many orientations display two sets of cleavage at 90o; others display a single

cleavage; acicular, tabular and bladed crystals are length slow

Diagnostic Properties: the right-angle cleavages, moderate relief and low birefringence

are characteristic, as are the length slow cross-sections; very difficult to distinguish from

celestite in thin-section

Occurrences/Associations: Most barite occurs in low-temperature hydrothermal vein and

replacement deposits where it is associated with galena, sphalerite, fluorite, pyrite, calcite and

other carbonates. Some barite occurs as concretions and veins in sedimentary rocks such as

limestone, mudrock and sandstone and as a cement in the latter. Rare barite occurs as a primary

igneous mineral in carbonatites associated with silica undersaturated rocks such as foid syenite.

Uses: Barite’s principal use to increase the density drilling muds used in the recovery of

petroleum from wells and which helps to prevent “blowouts”; also used as a source of barium in

the manufacture of glass, paper and ceramics products and in the production of rubber and

plastics; for x-ray images of the stomach and intestinal tract

BAUXITE: (technically a rock or soil type, but commonly discussed with minerals)

Composition: bauxite is typically a mixture of at least three minerals: gibbsite [Al(OH)3],

diaspore (AlOOH), and boehmite (AlOOH); in addition clay minerals, especially kaolinite,

limonite and hematite can be significant constituents of such mixtures as well; bauxite is

analogous to limonite mixtures for iron oxides and oxyhydroxides.

Crystal System: Gibbsite is monoclinic (2/m), whereas diaspore and boehmite are

orthorhombic (2/m/2m2/m):

Crystal Habit: bauxite mineral crystals are typically very small, sometimes capillary, and occur

in pisolitic, tubular and massive, earthy aggregates


Hardness: 2 ½ - 7; soft to hard; variable hardness in part due to hardness of

constituent minerals; Gibbsite = 2 ½ -3 ½, Boehmite = 3½ - 4, Diaspore = 6½ - 7; also

generally softer in less cohesive finer grained aggregates

Specific Gravity: 2.0 – 3.5; low to moderate; due in part to compactness (porosity) and

in part to the different specific gravities of the three minerals

Cleavage/Fracture: each mineral has cleavage, but it is generally not discernible in

crystal aggregates

Diaphaneity: typically somewhat to slightly translucent in aggregates

Colors: typically white, but also pale gray, green or brown; yellow brown where mixed

with limonite and reddish where mixed with hematite

Streak: white when mostly aluminum minerals

Luster: typically dull to pearly

Diagnostic Properties: The pisolitic habit, dull to pearly luster and white color (tinged

with yellow-brown or red) are highly suggestive, but the variability of bauxite mixtures

makes generalizations challenging; calcite and aragonite pisoids effervesce readily in

HCl, whereas bauxite does not

Microscopic Properties: the aluminum minerals in bauxite typically are colorless, have

moderate to high relief and low to moderate birefringence; typically, XRD methods are required

to identify them accurately

Occurrences/Associations: Bauxite occurs most commonly in severely leached soils (oxisols)

produced in warm, humid environments. In such soils, acidic waters leach silica from clay

minerals to produce concentrations aluminum oxides and hydroxides. These are commonly

associated with iron oxide and iron hydoxyoxide minerals and mineraloids including hematite,

and limonite and, less commonly, manganese oxides and hydroxides in the form of wad.

Uses: Bauxite deposits constitute the principal source for aluminum (Al) used in the manufacture

of household goods including cans, bottle caps, pots and pans, aluminum foil and table ware.

Aluminum finds widespread use in the transportation industry in alloys for automobiles, aircraft

and railway cars. Large amounts are used in electrical transmission lines, as well as in medical

packaging, baseball bats, gutters and aluminum siding. Aluminum oxide is used in the

production of various refractory materials, for example in the manufacture of spark plugs and

refractory bricks.

BERYL:

Composition: Be3Al2Si6O22; a cyclosilicate mineral; chromium (Cr+3

) substitutes for aluminum

(Al+3

); lithium (Li+1

) commonly substitutes for beryllium (Be+2

) or aluminum (Al+3

) with the

charge difference balanced by the incorporation of other +1 ions (Li+1

, K+1

, Na+1

, Cs+1

) in the

spaces inside rings (ring channels); water is also so incorporated in many examples; other

substitutions exist

Crystal System: Hexagonal (6/m2/m2/m): a1 = 9.20 – 9.27Å, a2 = 9.20 – 9.27Å, c = 9.19 –

9.25Å; α = 120o, β = 90

o, γ = 120

o

Crystal Habit: typically as six sided, columnar prismatic {10bar10} hexagonal crystals,

commonly with pinacoidal {0001} terminations; less commonly as tabular crystals with

hexagonal outlines; in divergent aggregates and radiating aggregates of acicular crystals


Hardness: 7½ - 8; very hard

Specific Gravity: 2.63 – 2.97; moderate; increases with increasing sodium, potassium

and cesium content

Cleavage/Fracture: generally not discernible; a weak basal cleavage exists


Colors: typically green, greenish blue, greenish yellow; also blue, yellow, colorless

Streak: white

Luster: vitreous

Diagnostic Properties: The common hexagonal crystals, high hardness and greenish

color are sufficient to identify beryl; quartz crystals are softer and typically possess

pyramidal terminations; apatite is softer


Refractive Indices: nω = 1.560 -1.610; nε = 1.557 – 1.599

Relief: low to moderate, positive


Birefringence: 0.003 – 0.009; very low to low; maximum first order yellow colors

Optic Sign: Uniaxial (–)

Other: tangential sections have hexagonal outlines and symmetrical extinction;

longitudinal sections are length fast with parallel extinction; long sections of tabular

crystals, perpendicular to the c-axis, are length slow, with parallel extinction; crystals

may be zoned

Diagnostic Properties: the very low birefringence, low relief, uniaxial (–) optics, lack of

recognizable cleavage and, where present, hexagonal cross-sections are diagnostic;

apatite is distinguished by its significantly higher indices of refraction and relief, its

“stippled” appearance in plane light and generally much clearer optic axis figures

Occurrences/Associations: Beryl most commonly occurs, sometimes in large crystals, in alkali

granite pegmatite where it is associated with tourmaline, muscovite-lepidolite, K- feldspar,

albite-rich plagioclase and quartz. It also occurs as an accessory mineral in many granitoids and

in foid syenite. Beryl also occurs in some high temperature hydrothermal vein and replacement

deposits and contact metamorphic rocks including skarn where it is associated with wolframite,

scheelite and cassiterite.

Uses: Beryl is the major source of beryllium (Be). Its low specific gravity and strength allows it

to be used for the production of light weight alloys for high-speed aircraft, missiles,

communications satellites and spacecraft. It is alloyed with copper to improve its strength and

hardness for use in springs, electrical contacts and many tools. Ultrathin beryllium foils are

essential for the reproduction of microscopic integrated circuits in the field of X-ray lithography.

Beryl is also an important gemstone, with emerald (green, Cr-bearing), aquamarine (blue-

green) and morganite (pink to red, Mn-bearing) being the principle varieties. High quality

emeralds are particularly desirable.

BIOTITE: (and PHLOGOPITE)

Composition: K(Mg,Fe)3AlSi3O10(OH)2; forms a solid solution series with phlogopite

[KMg3AlSi3O10(OH)2] which has more magnesium (Mg+2

) and less ferrous iron (Fe+2

) and with

annite [KFe3AlSi3O10(OH)2] which has more ferrous iron (Fe+2

) and less ferric iron (Mg+2

);

significant substitution of aluminum (Al+3

) for both silicon (Si+4

) and ferrous iron and/or

magnesium (Fe+2

and/or Mg+2

) also occurs; oxidized biotite, in which substantial ferric iron

(Fe+3

) has replaced ferrous iron (Fe+2

) with the simultaneous substitution of oxygen (O-2

) for

hydroxyl ion (OH-1

) to maintain charge balance are known as oxybiotite


o, γ = 90

o;

Crystal Habit: tabular (six-sided cross sections) to platy-scaly crystals; common in foliated

aggregates and as disseminated crystals


Hardness: on cleavage surfaces: 2½ - 3; soft, especially on cleavage surfaces

Specific Gravity: 2.8 – 3.2; moderate; lower end specific gravity for phlogopite

Cleavage/Fracture: one set of basal pinacoidal {001} cleavage; perfect

Diaphaneity: quite translucent to somewhat translucent; phlogopite may be nearly

transparent

Colors: typically dark brown to black or green; also reddish brown; phlogopite is

typically light, honey brown to tan or nearly colorless

Streak: white to gray

Luster: vitreous

Other: flexible and elastic in thin sheets; biotite alters to chlorite and to clay minerals

such as vermiculite, a hydrated version of biotite produced by weathering or

hydrothermal alteration, and to limonite

Diagnostic Properties: the dark color, along with the low hardness, tendency to split into

thin flakes or sheets along one perfect cleavage and the elasticity of such sheets are

diagnostic; phlogopite typically is a lighter shade of honey-brown to tan as a result of its

lower iron content; annite is black


Refractive Indices: nα = 1.565 – 1.625; nβ = 1.612 – 1.696; nγ = 1.613 – 1.696;

phlogopite has the lower refractive indices

Relief: low to moderate to moderate, positive; phlogopite has lower relief

Color/Pleochroism: strongly pleochroic in shades of yellow, pale green, light brown,

greenish brown, green, olive brown, yellow-brown, brown, reddish brown and dark

brown; phlogopite is colorless to pale yellow, pale brown, buff, brownish yellow, olive

green, salmon pink, reddish orange and pale red brown

Birefringence: 0.32 – 0.066; high to very high; maximum colors are third to even fourth

order, but are obscured somewhat by the strong coloring; phlogopite has somewhat

lower birefringence with high second to third order colors commonly; both minerals

display low birefringence when the cleavage is parallel to the stage, e.g., in grain mounts

Twinning: usually not discernable, but may occur

Optic Sign: Biaxial (–); 2Vx = 0-25o; low 2V; may appear uniaxial or nearly so;

phlogopite is similar with 2Vx = 0 – 15o

Other: “birdseye” maple appearance near extinction which occurs at small angles (0 –

9o) to the prominent cleavage; length slow parallel to cleavage, but difficult to see

because of deep colors; wavy extinction in bent flakes; hexagonal sections display

symmetrical extinction; phlogopite has similar characteristics and the length slow

property is easier to discern

Diagnostic Properties: the combination of a single perfect cleavage, strong colors and

pleochroism, birdseye extinction, near parallel extinction and biaxial (–) optics with a

small 2V are characteristic; distinguished from other micas by the strong colors and

pleochroism; phlogopite has paler colors, less intense pleochroism, an even smaller

maximum 2V and a generally different set of associations; biotite resembles

stilpnomelane, but the later lacks birdseye extinction, displays deeper yellow colors and

commonly forms a radial or sheaf-like crystal aggregate; green biotite differs from

chlorite by having much higher birefringence and by lacking anomalous interference

colors

Occurrences/Associations: Biotite is an abundant mineral in igneous, metamorphic and

sedimentary rocks. In igneous rocks, it is especially abundant in felsic rocks including

granitoids, pegmatite, aplite, syenite, foid syenite and equivalent volcanic rocks, sometimes as

oxybiotite. It also occurs as an accessory mineral in intermediate rocks such as diorite and in

some mafic/basic rocks including gabbro and norite, often by late-stage reaction with

hornblende. Biotite occurs widely in fairly low- to fairly high-grade pelitic metamorphic rocks

including hornfels, slate, phyllite, schist, and gneiss and in mafic metamorphic rocks including

amphibolite. Phlogopite, on the other hand, is a common accessory in ultramafic/ultrabasic

igneous rocks, where it is associated with pargasitic amphibole, pyroxenes and olivine, and in

calcareous metamorphic rocks produced by the metamorphism of impure dolostone and in some

skarns, where it is associated with calcite, dolomite, diopside, wollastonite, grossular-pyrope

garnet and forsterite-rich olivine.

Uses: Biotite is not of great economic value. Vermiculite, produced by the hydration of biotite,

is used as a soil conditioner for potting soil, as a component of insulation products and with

gypsum in sheet rock, wall board and other construction materials.

BORAX:

Composition: Na2B4O5(OH)4•8H2O;


o, γ =

90o

Crystal Habit: typically occurs in flattened, stubby prismatic to tabular-bladed crystals that

occur in divergent to reticulated aggregates; also in massive to granular aggregates


Hardness: 2 – 2½

Specific Gravity: 1.71; very low

Cleavage/Fracture: one orientation; pinacoidal {100}; perfect; there are two much less

discernable orientations not at 90o; prismatic {110}; fair


Colors: colorless to white; may have a gray to pale green or bluish tint

Streak: white


Other: possesses a sweet, alkaline taste; transparent crystals acquire a white, crumbly

coating (the mineral tincalconite) by dehydration in contact with the atmosphere; readily

soluble in water

Diagnostic Properties: distinguished by its crystal habit, sweet alkaline taste, white

coating, very low specific gravity and ready solubility in water


Refractive Indices: nα = 1.447; nβ = 1.469; nγ = 1.472



Birefringence: 0.025; moderate; maximum second order green to yellow colors

Twinning: may occur

Optic Sign: Biaxial (–); 2Vx = 39 - 40o; moderate; tincalconite is Uniaxial (+)

Other: possesses strong crossed dispersion and optic normal sections display anomalous

interference colors in shades of blue and brown

Diagnostic Properties: the combination of rather low refractive indices, moderate

negative relief, and moderate birefringence, strong crossed dispersion and abnormal

interference colors and the biaxial (–) optics with moderate 2V and single excellent

cleavage are diagnostic

Occurrences/Associations: Most borax forms by evaporation in enclosed saline lake in warm,

arid climates and as an efflorescent formed by soil water evaporation under similar conditions.

Commonly associated minerals include kernite, ulexite, colemanite, gypsum, anhydrite, halite

and a host of rare borate minerals.

Uses: The most important use of borax is in the manufacture of glass fibers for use in insulation

and textiles. Borax is also used in detergent soaps and as a flux in metallurgical processes such

as welding and smelting. One isotope of elemental boron, obtained from borax, is used in the

shields of atomic reactors and elemental boron is also used in fuels for rockets and automobiles

and in the high strength plastics used in aircraft manufacturing.

BORNITE:

Composition: Cu5FeS4; a sulfide mineral


90o, γ = 90

o; orthorhombic, rather than tetragonal, due to details of internal structure; isometric at

temperatures above 228oC (Nesse, 2000)

Crystal Habit: rare individual crystals are pseudocubic, octahedral or dodecahedral; most

commonly smaller crystals occur in massive to granular aggregates and as disseminated crystals


Hardness: 3; low side of moderate

Specific Gravity: 5.06 – 5.08; high

Cleavage/Fracture: very poor; so irregular (uneven) fracture generally seen

Diaphaneity: opaque

Colors: brownish bronze on fresh surfaces; rapidly tarnishes to iridescent colors

including purple and blue tints which give rise to the name “peacock ore”

Streak: gray black

Luster: metallic on fresh surfaces; submetallic on tarnished surfaces

Other: commonly alters by oxidation to copper carbonates such as malachite and azurite

and copper oxides such as cuprite; supergene enrichment causes alteration to chalcocite

and covellite; the iron may alter to siderite or limonite (goethite)

Diagnostic Properties: bronze color helps to distinguish it from minerals such as

chalcopyrite and chalcocite; color and softness distinguish it from pyrite and

marcasite; lack of magnetism distinguishes it from pyrrhotite; peacock ore iridescence

distinguishes it from all of these

Microscopic Properties: opaque; yellow in reflected light with pleochroism

Occurrences/Associations: Bornite is widely distributed in hydrothermal vein and replacement

deposits where it is associated with chalcopyrite, chalcocite, covellite, galena, sphalerite,

pyrrhotite, pyrite, tetrahedrite, and silver sulfides and a variety of gangue minerals. Bornite also

occurs with chalcocite and covellite in the zone of supergene enrichment. It occurs less

abundantly as disseminated crystals in some skarn deposits produced by contact metamorphism,

in pegmatites and in some mafic/basic igneous rocks such as gabbro.

Uses: Bornite is an important ore of copper (Cu) whose primary use is in the manufacture of

electrical wires and switches. Copper is also used in alloys: with zinc in brass, with copper and

tin in bronze and with copper, zinc and nickel in German silver.

BRUCITE:

Composition: Mg(OH)2; a hydroxide mineral; some iron (Fe+2

) and Manganese (Mn+2

) may

substitute for magnesium (Mg+2

)

Crystal System: Hexagonal (Trigonal) [bar32/m]: a1 = 3.14Ǻ, a2 = 3.14Ǻ, c = 4.77Ǻ; α =

120o, β = 90

o, γ = 120

o

Crystal Habit: crystals are platy to tabular; typically in foliated, “swirled”, or massive to

granular aggregates; rarely fibrous


Hardness: 2½; soft

Specific Gravity: 2.39; low-moderate

Cleavage/Fracture: one orientation of basal, pinacoidal {0001} cleavage; perfect; often

not discernible in crystal aggregates

Diaphaneity: somewhat to slightly translucent in aggregates

Colors: commonly white; often with a green or gray tint; less commonly bluish or

brownish

Streak: white

Luster: waxy to pearly in most aggregates; vitreous in crystals

Other: alters to serpentine, talc and hydrated magnesite

Diagnostic Properties: distinguished from talc by brucite’s superior hardness and lack of

greasy feel; from micas by the fact that brucite’s folia are not elastic; from chlorite and

most serpentine by brucite’s paler colors


Refractive Indices: nα = 1.559 – 1.590; nε = 1.580 – 1.600; increase with iron content

Relief: low-moderate to moderate, positive


Birefringence: 0.010 – 0.021; low to moderate; maximum colors upper first to lower

second order

Twinning: none

Optic Sign: Uniaxial (+)

Other: cross-sections of plates are length fast; may show anomalous first order bluish or

reddish-brown colors

Diagnostic Properties: the combination of length fast scaly to platy crystals in swirled

aggregates is distinctive; the lower birefringence and deep brown abnormal interference

colors distinguish brucite from talc; the serpentine minerals that resemble brucite have

scales that are length slow and typically possess lower birefringence; muscovite is

biaxial (–), albeit with a small 2V, and has very different associations than these

ultramafic/magnesian/calcareous metamorphic minerals

Occurrences/Associations: Brucite is typically a metamorphic mineral formed by the alteration

of other magnesium-bearing minerals including periclase, dolomite, olivine and pyroxene. It is

most commonly formed by hydrothermal, contact or regional metamorphism of

ultramafic/ultrabasic rocks such as peridotites and in Mg-bearing marbles where it is associated

with dolomite, magnesite, serpentine, talc and chlorite. Brucite also occurs with talc, serpentine

and magnesite in veins that cross-cut serpentinized peridotite bodies.

Uses: Brucite is mined on a small scale for use as a flame retardant in the manufacture of paper

and plastics and is also used in furnaces in the manufacture of steel. Synthetic brucite is

sometimes produced for these purposes.

CALCITE:

Composition: CaCO3; a carbonate; polymorphic with aragonite; isostructural with magnesite,

siderite smithsonite, and rhodochrosite, all members of the rhombohedral (calcite) group of

carbonate minerals; magnesium (Mg+2

) may replace a significant, but limited, amount of calcium

(Ca+2

); calcite with more than 4% magnesium (Mg+2

) replacing calcium (Ca+2

) is called high-

magnesium calcite; substantial manganese (Mn+2

) may also replace calcium (Ca+2

)

Crystal System: Hexagonal (Rhombohedral); (bar32/m): a1 = 4.99Ǻ, a2 = 4.99Ǻ, c = 17.1; α =

120o, β = 90

o, γ = 120

o

Crystal Habit: crystals common as stubby prismatic scalenohedra {21bar31} and slender prisms

{01bar10} in combination with rhombohera {10bar10} in divergent clusters and massive to

granular aggregates; also acicular crystals in fibrous habits in ooids and shells; rare as isolated

rhombohedral crystals


Hardness: 3


Cleavage/Fracture: 3 orientations not 90o

(74.9o and 105.1

o); rhombohedral {10bar1l};

perfect


Colors: variable (allochromatic); typically colorless to white; also gray, yellow, blue,

green, brown

Streak: white

Luster: vitreous; like all minerals, dull/earthy if in very fine grained aggregates

Other: Effervesces in cold, dilute hydrochloric acid (HCl); transparent specimens

(Iceland spar) exhibit obvious double refraction

Diagnostic Properties: Calcite is recognized by its fine rhombic cleavage, its hardness

(3) and its effervescence in cold, dilute HCl. Dolomite does not readily effervesce in

cold, dilute HCl and aragonite does not have rhombic cleavage and has a higher SG.


Refractive Indices: nω = 1.668; nε = 1.486

Relief: variable; from low-moderate negative to moderately-high positive; sections in

which both nω and nε are visible produce a “twinkling” effect as the relief changes when

stage is rotated under plane light


Birefringence: 0.172; very high-extreme; high order faded colors; “creamy white”

Twinning: rhombohedral mechanical twins common


Other: extinction is symmetrical with respect to intersecting cleavages; calcite twins tend

to parallel the long diagonals between cleavage traces; strained crystals (e.g., from

marble and calcsilicate rocks) may be biaxial with a small 2V (< 15o)

Diagnostic Properties: the combination of extremely variable relief leading to a

“twinkling effect”, the extreme birefringence, the rhombohedral cleavage, and the

uniaxial (–) optics are characteristic of rhombohedral carbonates such as calcite; calcite is

distinguished from dolomite only with difficulty; dolomite tends to occur more

commonly as euhedral rhombohedra in sedimentary rocks, is less commonly twinned,

possesses twins that parallel both the long and short diagonals of cleavage traces and

tends to be stained with iron oxides more commonly because iron (Fe+2

) substitutes

easily for the magnesium (Mg+2

) in dolomite; calcite is distinguished from aragonite by

its rhombohedral cleavage and its uniaxial optics

Occurrences/Associations: Calcite is an abundant mineral in sedimentary rocks where it is the

major component of shells and therefore of most limestone. It is also a significant component of

calcareous detrital sediments such as marl, calcareous shale and calcareous sandstone and is an

important cement in detrital sediments including sandstone. Calcite also occurs in where

groundwater discharges in caves and around springs where loss of CO2 from solution lowers

acidity and causes it to precipitate. Calcite is also a significant mineral in some soils, especially

“caliche” aridosols formed in warm, arid climates, often forming hard calcrete horizons in such

soils. Calcite is also abundant in metamorphic rocks as the major component of marble and

calcsilicate rocks including skarn. Calcite is an important constituent of many hydrothermal vein

deposits. Much less commonly, calcite occurs as a primary mineral in silica undersaturated

alkalic igneous rocks such as foid syentite and in rare carbonate igneous rocks called

carbonatites. It also occurs as a secondary mineral in many igneous rocks where it is a product

of the alteration of calcium-bearing plagioclase.

Use: Calcite, in the form of impure limestone, is the major raw material for the lime used cement

products and mortars such as concrete which are widely used in construction. Lime is also used

to whitewash structures and draw lines on playing fields. Cut blocks of limestone and marble,

composed largely of calcite, are used as dimension stone in many construction projects.

CASSITERITE:

Composition: SnO2;

Crystal System: Tetragonal (4/m2/m2/m): a1 = 4.59Ǻ, a2 = 4.59Ǻ, c = 2.96Ǻ; α = 90o, β = 90

o,

γ = 90o

Crystal Habit: individual crystals are rare stubby to elongate prisms{110} with dipyramid

{111} terminations; commonly in massive to granular aggregates and as disseminated crystals or

grains; also in colloform (botryoidal) masses of radiating acicular-capillary crystals called “wood

tin”



Specific Gravity: 6.9 – 7.1; very high

Cleavage/Fracture: not generally discernible

Diaphaneity: barely to slightly translucent

Colors: typically yellowish brown to reddish brown to brownish black; also gray, reddish

Streak: brownish to grayish white

Luster: submetallic to adamantine or splendant

Diagnostic Properties: The yellow brown to brown black color; adamantine to

submetallic luster and botryoidal habit are characteristic; rutile is similar, but has a lower

specific gravity; more of a reddish black color and commonly occurs in striated

prismatic to acicular crystals rather than botryoidal aggregates


Refractive Indices: nω = 1.992 – 1.997; nε = 2.091 – 2.093

Relief: extremely high, positive

Color/Pleochroism: many colors, commonly color zoned in shades of yellow, orange-

red, red, brown or gray; colors may display a mottled or “splotchy” distribution

Birefringence: 0.098 – 0.100; very high; colors often masked by the mineral’s color

Optic Sign: Uniaxial (+); the high refractive index makes it difficult to obtain clear

figures

Other: length slow, but color and relief may make determination difficult; extinction is

parallel to longitudinal sections

Diagnostic Properties: the often zoned or splotchy yellow-orange-red-brown colors,

the extremely high relief, and the uniaxial (+) optics serve to distinguish cassiterite from

most other minerals; cassiterite resembles rutile, but the latter possesses even higher

relief, two distinct cleavages at right angles and rarely occurs in radial-fibrous or

colloform aggregates

Occurrences/Associations: Cassiterite is a widespread, but generally scarce accessory mineral

that occurs in many granitoids (especially S-type granitoids) and pegmatites. It also forms in

high temperature hypothermal veins and contact metamorphic rocks adjacent to such granitoids

where it is associated with topaz, tourmaline, fluorite, apatite, wolframite, molybdenite and

arsenopyrite. Because it is highly resistant to decomposition during weathering and diagenesis,

cassiterite is concentrated in detrital sediments derived from such source rocks, sometimes

forming placer deposits that are mined commercially.

Uses: Cassiterite is the principle ore of tin (Sn) used in the manufacture of “tin” cans which are

steel electroplated with tin and in much aluminum cookware which is also plated with tin. Tin is

also alloyed with copper in the production of bronze and with antimony and lead in the

production of “lead solder” for joining pipes and electrical junctions. Tin is widely used in the

production of capacitors and resisters used in integrated electronic circuits.

CELESTITE:

Composition: SrSO4; a sulfate mineral; isostructural with barite; a complete solid solution

series is possible and Ba+2

typically substitutes for Sr+2

to some degree producing variable

specific gravity, refractive indices and birefringence


90o, γ = 90

o

Crystal Habit: individual crystals are typically tabular to bladed; less commonly prismatic; most

commonly reticulated aggregates of tabular to bladed crystals; also in divergent aggregates of

prismatic to bladed crystals; also occurs in granular or massive aggregates and in radiating

fibrous aggregates of acicular-capillary crystals



Specific Gravity: 3.9 – 4.0; moderately high

Cleavage/Fracture: four cleavage orientations; one pinacoidal {001}, perfect cleavage

is at right angles to two prismatic {210} very good cleavages that are not at right angles

to each other (78o and 102

o); a fourth cleavage is pinacoidal {010} fair, so not readily

discerned


Colors: commonly pale blue, colorless or white; may be gray or pale red, green or

brown

Streak: white

Luster: vitreous

Diagnostic Properties: Celestite is distinguished from carbonates with elevated specific

gravity such as witherite and strontianite by its lack of effervescence in HCl and its three

orientations of cleavage; it is similar to barite, but has a lower specific gravity and is

more typically a bluish color





Birefringence: 0.008 – 0.009; low; maximum color is first order gray-yellow

Twinning: rare; usually absent

Optic Sign: Biaxial (+); 2Vz = 50 - 51o; moderate

Other: many orientations display two sets of cleavage at 90o; others display a single

cleavage; acicular, tabular and bladed crystals are length slow

Diagnostic Properties: the right-angle cleavages, moderate relief and low birefringence

are characteristic; celestite is very difficult to distinguish from barite in thin-section

Occurrences/Associations: Celestite typically occurs in sedimentary rocks such as limestone or

sandstone where it is found as disseminated crystals and as crystal aggregates that line cavities. It

also occurs in low-temperature hydrothermal veins associated with fluorite, galena and sphalerite

and in some marine evaporite deposits where it is associated with gypsum, anhydrite, halite and

sulfur.

Uses: The major ore of strontium (Sr) used to plate the screens of cathode ray tubes to prevent x-

ray leakage and therefore in television and computer screens; other uses include the production

of ceramics and fluorescent lights and of the red colors in flares, fireworks and tracer bullets

CERUSSITE:

Composition: PbCO3; a carbonate; isostructural with aragonite, witherite and

strontianite, all members of the orthorhombic (aragonite) group of carbonate minerals

Crystal System: Orthorhombic (2/m2/m2/m): a = 5.15Ǻ, b = 8.47Ǻ, c = 6.11Ǻ; α =

90o, β = 90

o, γ = 90

o

Crystal Habit: typically tabular-bladed crystals; reticulated aggregates common;

penetration twins commonly produce columnar pseudohexagonal crystals; also in

granular to compact massive aggregates



Specific Gravity: 6.55; very high

Cleavage/Fracture: two cleavages not 90o; prismatic {110}, good; another indistinct


Colors: colorless, white or gray

Streak: white

Luster: vitreous to adamantine

Other: effervesces in warm, dilute nitric acid (HNO3)

Diagnostic Properties: The high specific gravity, adamantine luster, effervescence

in nitric acid and association with other oxidized zone minerals are useful in

recognizing cerrusite



Relief: very high-extreme, positive


Birefringence: 0.273; extreme; very high order faded colors (“creamy white”)

Twinning: alternating penetration twins common

Optic Sign: Biaxial (–); 2Vx = 9o (very small 2V)

Diagnostic Properties: cerussite is recognized by its very high refractive indices and

relief, the biaxial negative optics with a very small 2V, the two prismatic cleavages and,

where present, the psuedohexagonal crystals; the very high refractive indices and relief

and the lack of “twinkling” as the stage is rotated under plane light distinguish cerrusite

from the other orthorhombic carbonate minerals (aragonite, strontianite and witherite)

which have much lower indices and relief

Occurrences/Associations: Cerussite forms in the oxidized zone of hydrothermal vein and

replacement deposits that contain galena. It is formed by the reaction of acidic, carbonate rich,

meteoric ground water with galena (PbS) to cause oxidation. What cerrusite is associated with

depends on the other primary sulfide minerals, but common associates include anglesite, limonite

(goethite), smithsonite, hemimorphite, malachite, azurite, and chrysocolla.

Uses: Cerussite is an ore of lead used in lead-acid batteries. When alloyed with tin and antimony

lead is used to produce electrical solder. Lead was formerly used as a gasoline additive to raise

octane levels and was a major component of paints. Because of the deleterious health effects of

lead, these uses have largely been discontinued.

CHABAZITE:

Composition: Ca2Al2Si4O12•6H2O; a tectosilicate mineral; a zeolite mineral; some substitution

of sodium (Na+1

) and potassium (K+1

) for calcium (Ca+2

) is balanced by substitution of silicon

(Si+4

) for aluminum (Al+3

) commonly occurs

Crystal System: Triclinic (ī): a = 9.40 – 9.45Å, b = 9.40 – 9.45Å, c = 9.40 – 9.45Å; α = 94.2 –

94.3o, β = 94.1 – 94.4

o, γ = 94.1 – 94.5

o; crystals are pseudo-rhombohedral/hexagonal

Crystal Habit: occurs as equant, pseudo-rhombohedral crystals with angles close enough to 90o

to resemble cubes on first inspection; often as granular aggregates of such crystals or as more

finely-crystalline massive aggregates or disseminated crystals


Hardness: 4½; moderate


Cleavage/Fracture: three sets of “rhombohedral” cleavage at ~ 86o and 94

o; good,

“psuedocubic”


Colors: typically colorless to white; also shades of yellow, pink and red

Streak: white

Luster: vitreous

Diagnostic Properties: The white color, intermediate hardness and pseudo-

rhombohedral crystals that resemble cubes are diagnostic; finely crystalline aggregates

generally require XRD analysis for accurate identification



Relief: low to moderate, negative


Birefringence: 0.02 -0.10; low; maximum colors first order grays and white

Twinning: penetration twins common

Optic Sign: Uniaxial (+) or (-) or Biaxial (+) or (–); 2V = 0 – 20o; none for uniaxial

varieties, small 2V for biaxial varieties

Diagnostic Properties: the pseudo-rhombohedral (nearly cubic) cleavage with 94o and

86o intersection angles and uniaxial to biaxial optics with small 2V distinguish chabazite

from most other common zeolite minerals; heulandite also possesses low birefringence, is

optically biaxial (+) and can possess a small 2V, but is never optically (–) and possesses

only one cleavage; analcime is isotropic and other zeolites possess a larger 2V,

different cleavage and somewhat higher birefringence

Occurrences/Associations: Chabazite is a relatively widespread zeolite mineral. Coarse

crystals are especially common in cavities, including vesicles, in mafic/basic rocks such as basalt

and, less commonly, in intermediate volcanic rocks such as andesite. Finely crystalline

chabazite occurs abundantly in altered felsic volcanic rocks, especially in volcanic ash beds

deposited in lakes/oceans or altered by hydrothermal or groundwater circulation systems.

Chabazite is also reported from alkaline soils formed in desert environments and in some contact

metamorphic aureoles.

Uses: Chabazite, like most natural and synthetic zeolites, is used, after heating and dehydration,

to produce materials that remove water vapor from carbon dioxide, from refrigerants such as

Freon and from many organic chemicals used in industrial applications. It is also used as a

molecular sieve used to separate molecules of different sizes as in separating nitrogen from air to

produce nearly pure oxygen gas. Because zeolites can selectively absorb selected molecules,

they are widely used in applications that include reducing the hardness of water for cleaner

laundry by the removal of calcium, removing heavy metals from mine waters and industrial

waste and radioactive isotopes from nuclear waste, in the treatment of wastewater by the removal

of undesirable dissolved species such as ammonia and in the cleaning up of oil spills.

CHALCEDONY: see CHERT

CHALCOCITE:

Composition: Cu2S; a sulfide mineral

Crystal System: Orthorhombic (mm2): a = 11.82Ǻ, b = 27.05Ǻ, c = 13.42Ǻ; α = 90o, β = 90

o,

γ = 90o; hexagonal polymorph stable at high temperature

Crystal Habit: small tabular crystals with hexagonal outlines occur; more typically in massive,

finely crystalline aggregates


Hardness: 2½ - 3; soft to lowest moderate


Cleavage/Fracture: cleavage indistinct; conchoidal fracture

Diaphaneity: opaque

Colors: lead gray; tarnishes sooty black

Streak: gray black

Luster: metallic on fresh surfaces

Other: sectile; though brittle if crushed

Diagnostic Properties: The lead gray color, lack of cleavage, sectility and black tarnish

are characteristic

Microscopic Properties: opaque; white in reflected light

Occurrences/Associations: Chalcocite commonly occurs in the zone of supergene enrichment

where groundwater concentrates metal from minerals in hydrothermal vein and replacement

deposits. In such deposits, it is commonly associated with covellite and several rarer copper and

silver sulfide minerals. Chalcocite also occurs sparingly in some hydrothermal veins.

Uses: Chalcocite is a rich ore of copper, but scarce enough to not be an important source. Copper

used primarily in electrical wiring and switches and in the manufacture of alloys such as brass

(with zinc), bronze (with tin and zinc) and German silver (with zinc and nickel) used for various

implements.

CHALCOPYRITE:

Composition: CuFeS2; a sulfide mineral

Crystal System: Tetragonal (bar42m): a1 = 5.28Ǻ, a2 = 5.28Ǻ, c = 10.40Ǻ; α = 90o, β = 90

o, γ

= 90o

Crystal Habit: the dominant forms are fairly equant tetragonal disphenoids {112} that resemble

isometric tetrahedrons; may form druse-like divergent crystal aggregates; more commonly

smaller crystals occur in massive to granular aggregates and disseminated crystals


Hardness: 3½ to 4

Specific Gravity: 4.1 -4.3; high

Cleavage/Fracture: very poor; so irregular (uneven) fracture

Diaphaneity: opaque

Colors: brass-yellow; tarnishes to bronze or iridescent colors

Streak: greenish black

Luster: metallic on fresh surfaces; submetallic on tarnished surfaces

Other: commonly alters by oxidation to copper carbonates such as malachite and azurite

and copper oxides such as cuprite; supergene enrichment causes alteration to chalcocite

and covellite; the iron may alter to siderite or limonite (goethite)

Diagnostic Properties; distinguished from pyrite by being softer and by greenish tint to

deeper brass yellow color; from pyrrhotite by deeper yellow color and lack of

ferrimagnetism

Microscopic Properties: opaque; yellow in reflected light with pleochroism

Occurrences/Associations: Chalcopyrite is widely distributed in low (epithermal-telethermal) to

high temperature (hypothermal and contact metamorphic) hydrothermal vein and replacement

deposits where it is associated with galena, sphalerite, pyrrhotite, pyrite, bornite, tetrahedrite,

stibnite, silver sulfides and wolframite and a variety of gangue minerals. It is also a common

mineral in many skarn deposits produced by contact metamorphism. It also occurs in

mineralized igneous rocks of the granitoid clan (e.g., in “porphyry copper” deposits) and in some

mafic/basic igneous rocks such as gabbro.

Uses: Chalcopyrite is the major ore of copper (Cu) whose primary use is in the manufacture of

electrical wires and switches. Copper is also used in alloys: with zinc in brass, with copper and

tin in bronze and with copper, zinc and nickel in German silver.

CHERT: (aka CHERT GROUP including CHALCEDONY)

Composition: SiO2; a tectosilicate mineral; a microcrystalline aggregate of quartz,; chert

varieties, largely based on color, include chert (white to light gray), flint (medium-dark gray to

black), jasper (red, brown or yellow-brown, the latter sometimes called sard) and prase (green);

an important subvariety of chert is chalcedony which is also a microcrystalline aggregate of

quartz (and perhaps morganite), but possesses a micro-fibrous to sheaf-like crystal habit; color

banded chalcedony is called onyx (where banding is flat) and agate (where banding is

concentric); it should be pointed out that not all concerned agree with the above definitions for

the varieties of microcrystalline silica, but we think this is a workable approach; colors generally

result from microcrystalline impurities; both chert and chalcedony are members of the silica

group that includes quartz, tridymite, coesite and stishovite

Crystal System: Hexagonal (32): a1 = 4.91Å, a2 = 4.91Å, c = 5.41Å; α = 90o, β = 120

o, γ = 90

o

Crystal Habit: microcrystals are typically equant and occur in massive aggregates of

microcrystals; in chalcedony the crystals are micro-fibrous to radiating “sheaf-like” and may

occur as stellated aggregates in spherulites




Cleavage/Fracture: none; excellent conchoidal fracture

Diaphaneity: somewhat translucent to barely translucent; chalcedony can be quite

translucent

Colors: variable, see above; white to light gray (chert), medium-dark gray to black

(flint), red, brown or yellow-brown (jasper), green (prase); chalcedony is typically light

to medium gray, but can also be green (chrysoprase), red (carnellian) and yellow-brown

(sard); multiple colors commonly occur in a single specimen

Streak: white

Luster: dull to waxy (especially in chalcedony)

Diagnostic Properties: the hardness, excellent conchoidal fracture, dullish luster and

distinctive colors are characteristic


Refractive Indices: nω = 1.544; nε = 1.553; chalcedony may possess lower indices

Relief: low, positive


Birefringence: 0.009; low; maximum first order grays and white; yellow only if thin-

section is too thick

Optic Sign: Uniaxial (+); but figures are difficult to obtain in finely crystalline material

Other: fibers in chalcedony may be length fast or length slow depending on

crystallography of the elongation; these may alternate in some specimens

Diagnostic Properties: the combination of low refractive indices and low positive relief,

microscopic crystals with low birefringence (“salt and pepper” appearance) and lack of

cleavage (good conchoidal fracture) are distinctive; large microcrystals may display

uniaxial (+) optics; the radial-fibrous habit of chalcedony is also distinctive; distinction

between chert and other fine-grained silica minerals such as cristobalite and tridymite

requires advanced techniques such as XRD

Occurrences/Associations: Chert is an abundant component of many sedimentary rocks. Much

chert forms during diagenesis as nodules and irregular beds formed by the replacement of

carbonate minerals in limestone (including chalk) and dolostone. Other chert, typically bedded,

forms by the diagenetic conversion of the opalline shells of radiolaria and diatoms into chert. In

many cases, fossil remains are preserved in the chert and yield clues to its origin. Chert,

especially in the form of flint and jasper is also an important constituent of Precambrian banded

iron formations (BIF). Chert, like other varieties of quartz, is quite resistant to decomposition

during weathering and is therefore a fairly common constituent of detrital sedimentary rocks

including breccia, conglomerate and sandstone.

Uses: Chert family varieties have been widely used as building stone, e.g., in flint churches,

fireplaces and boundary walls. Jasper and prase are used for jewelry and as decorative stone.

Microcrystalline quartz sparks easily when struck against steel and is used in “flints” in lighters,

stoves and barbecues (and formerly in “flintlock” rifles). In former times, because of its

hardness and excellent conchoidal fracture which produces sharp edges, chert was used

extensively in the creation of arrowheads, spear points, scrappers, hand axes and drills. Chert is

a minor source of quartz for the manufacture of glass products and is used in abrasive materials

such as sandpapers and cleansers.

CHLORITE:

Composition: (Mg,Fe,Al)3(Si,Al)4O10(OH)2•(Mg,Fe,Al)3(OH)6; a phyllosilicate mineral with a

T-O-T + O structure; when finely-crystalline (< 4μm), is considered a clay mineral; very variable

composition, as indicated by the formula; recognized subvarieties of chlorite include

magnesium-rich and silica-poor clinochlore, magnesium-rich and silica-rich pennine and

aluminum-rich prochlorite; manganese (Mn+2

), chromium (Cr+3

) are common substitutions

Crystal System: Monoclinic (2/m): a = 5.2 – 5.3Å, b = 9.2 – 9.3Å, c = 14.3Å; α = 90o, β = 97

o,

γ = 90o; some chlorite is triclinic

Crystal Habit: small, scaly-platy to tabular crystals with hexagonal outline; however, typically

finely crystalline foliated aggregates; also massive to granular aggregates where crystals are

randomly oriented; occasionally as spherulitic to oolitic aggregates


Hardness: 2 – 3; soft to borderline moderate

Specific Gravity: 2.6 – 3.3; moderate; increases with iron content

Cleavage/Fracture: one orientation of basal pinacoidal {001} cleavage; excellent; often

not discernable in finely crystalline aggregates

Diaphaneity: translucent to nearly transparent in thin sheets

Colors: typically various shades of light to dark green; also yellow green; rarely pinkish

(when manganese rich)

Streak: white to pale green

Luster: vitreous to pearly; dull in some finely crystalline aggregates

Other: thin sheets are flexible, but not elastic; may alter to vermiculite and other clay

minerals and/or limonite; commonly replaces biotite and other ferromagnesian minerals

Diagnostic Properties: The green color, flexible, but inelastic sheets and low hardness

are distinctive and the common occurrence in metamorphic rocks helpful



Relief: moderate to moderately high, positive

Color/Pleochroism: slightly pleochroic in pale green, green, yellow green, brownish

yellow and/or greenish brown

Birefringence: 0.000 – 0.010; low; typically first order grays and white; anomalous

colors in shades of blue, violet or brown are common

Twinning: typically not discernable, but multiple twins do occur

Optic Sign: most commonly Biaxial (+), also Biaxial (–); 2Vz = 0 – 40o; 2Vx = 0 -60

o;

very low to moderate 2V

Other: cleavage traces length fast or length slow; extinction angles with respect to

cleavage are small (< 10o) and may be nearly parallel to it; unlike micas, there is no

“birdseye” extinction

Diagnostic Properties: the combination of low-moderate positive relief, one excellent

cleavage, slight pleochroism in shades of pale green and relatively low birefringence are

characteristic; the abnormal interference colors, when present, are helpful; talc is

generally colorless and has much higher birefringence; serpentine is similar in many

respects, but tends to have different crystal habits (fibrous or sheaf-like), displays

cleavage less often and is rarely biaxial (+)

Occurrences/Associations: Chlorite is especially abundant in low to medium grade pelitic and

mafic metamorphic rocks produced by regional metamorphism in the greenschist (so named

because of the abundance of chlorite and other green minerals) and epidote-amphibolite facies.

It is a common mineral in contact metamorphic rocks of similar composition produced in the

albite-epidote hornfels facies. It is also an important product of the weathering of ferromagnesian

silicate minerals and therefore an important constituent of soils and of fine-grained detrital

sediments such as shale and mudstone. Chlorite is also a common deuteric alteration product of

other ferromagnesian silicate minerals such as biotite, hornblende and pyroxenes in igneous

rocks.

Uses: Chlorite does not have great economic value. However, microcrystalline chlorite is an

important constituent of clay which is used in the production of bricks, ceramics, absorbents

(such as cat litter), cosmetics and refractory materials.

CHLORITOID:

Composition: (Fe,Mg,Mn)2(Al,Fe)Al3O2(SiO4)2(OH)4; a nesosilicate (orthosilicate) mineral

Crystal System: Monoclinic (2/m) and Triclinic (ī): a = 9.50Å, b = 5.50Å, c = 18.22Å; α =

90o, β = 102

o, γ = 90

o

Crystal Habit: individual crystals are platy with roughly hexagonal outlines; often in foliated

aggregates; may also occur as thick tabular crystals and in massive to granular aggregates or as



Hardness: 6 ½; hard

Specific Gravity: 3.5 -3.8; moderate plus

Cleavage/Fracture: one basal pinacoidal {001} cleavage; good, less perfect than in

micas and chlorite

Diaphaneity: slightly to somewhat translucent

Colors: typically greenish gray, green or greenish black

Streak: white to pale gray green

Luster: pearly, especially on cleavage surfaces

Other: brittle, commonly poikiloblastic; may show “hourglass structure”; alters to

chlorite, sericite and clay minerals

Diagnostic Properties: The high hardness, brittleness and poorer cleavage can be used to

distinguish chloritoid from chlorite in coarse material; in fine-material the distinction is

more difficult




Color/Pleochroism: typically somewhat pleochroic; colorless to green, gray green, olive

green, blue gray, indigo, blue green, greenish yellow, yellow, greenish brown

Birefringence: 0.007 – 0.012; low; maximum colors in first order gray to yellow; higher

in a few compositions with first order reds; often with anomalous birefringence

Twinning: lamellar twins common; hourglass twins also occur

Optic Sign: Biaxial (+); 2Vz = 36 -72o; when monoclinic

Biaxial (–); 2Vx = 55 -88o; when triclinic

Other: extinction angles with respect to cleavage are 7 – 18o, larger than in most micas

and chlorite; may be zoned with darker centers; long sections, parallel to cleavage, are

length fast

Diagnostic Properties: chloritoid may resemble chlorite, but has higher relief, larger

extinction angles, often different pleochroic colors and somewhat higher birefringence; it

lacks the near parallel, birdseye extinction and small 2V of biotite and other micas

Occurrences/Associations: Chlorotoid is a common mineral, often occurring as porphyroblasts,

in low- to medium-grade pelitic rocks such phyllite and schist formed by regional metamorphism

in the greenschist, amphibolite and blueschist facies. It is commonly associated with chlorite,

muscovite, biotite, garnet, staurolite, glaucophane, and lawsonite and muscovite. It is also

reported from contact metamorphic metapelites.

Uses: Chloritoid is not an economically valuable mineral.

CHROMITE:

Composition: FeCr2O4; an oxide; a spinel group (XY2O4 group) mineral; forms a complete

solid solution series with magnesiochromate (MgCr2O4) and generally contains some magnesium

in substitution for iron

Crystal System: Isometric (4/mbar32/m): a1 = 8.38Ǻ, a2= 8.38Ǻ, a3 = 8.38Ǻ; α = 90o, β = 90

o,

γ = 90o

Crystal Habit: occurs as small, equant octahedral {111} crystals; sometimes modified by cubic

{001} faces; more commonly as massive to granular crystal aggregates or disseminated crystals



Specific Gravity: 5.1; high; less if substantial magnesium is present

Cleavage/Fracture: none

Diaphaneity: nearly opaque

Colors: black

Streak: brown

Luster: metallic to submetallic (“pitchy”) if substantial magnesium is present

Other: simple or multiple twins occur

Diagnostic Properties: The brown streak, black color and pitchy luster are characteristic;

the hardness and high specific gravity useful confirmation; magnetite is magnetic and has

a black streak and more metallic luster


Refractive Index: n = 1.90 – 2.12

Relief: very high, positive

Birefringence: none, isotropic

Color: nearly opaque; dark brown on thin edges; subdued brownish gray in reflected

light

Optic Sign: none; isotropic

Diagnostic Properties: chromite resembles magnetite in thin-section, as both possess

octahedral crystals with their rhomboid cross-sections, but the magnetite is

completely opaque, whereas chromite is not; the brown edges under plane light are

distinctive

Occurrences/Associations: The vast majority of chromite occurs as an accessory mineral in

ultramafic/ultrabasic and mafic/basic igneous rocks including dunite, peridotite, pyroxenite and

gabbro. Thick layers rich in chromite are produced by magmatic differentiation/segregation

processes, especially in the ultramafic portions of large layered gabbroic intrusions (LLGI’s).

Smaller amounts of chromite occur as placers in some detrital sediments.

Uses: Chromite is the principle ore of chromium (Cr) used in stainless steel and corrosion

resistant electroplating for steel products. Chromium is used in a wide variety of industrial

processes as a refractory material and as a catalytic agent. Chromium, in different oxidation and

coordination states, is widely used as a pigmenting agent. Chromium is a toxic, cancer-causing,

heavy metal, especially in its hexivalent (Cr+6

) state. This toxicity permits it to be used to protect

and preserve wood products and paper from insects and fungi. Chromium is used in the tanning

of leather and trace amounts are used in dietary supplements.

CHRYSOTILE: see SERPENTINE

CINNABAR:

Composition: HgS; a sulfide mineral

Crystal System: Hexagonal (32): a1 = 4.146Ǻ, b1 = 4.146Ǻ, c = 9.497Ǻ; α = 120o, β = 90

o, γ =

120o; high temperature form is an isometric {bar43m} polymorph

Crystal Habit: crystals are typically small rhombohedra β {10īl} with thick tabular or stubby

prismatic habits; more typically in vein fillings and fine massive to granular aggregates and

drusy coatings and encrustations or disseminated as separate crystals; isometric crystals are

commonly tetrahedral {10ī}


Hardness: 2½; soft

Specific Gravity: 8.1; very high, but difficult to discern in disseminated cinnabar

Cleavage/Fracture: 3 sets not at 90o; rhombohedral {10ī1}, but commonly not visible

in massive to granular aggregates


Colors: bright vermillion red, with brownish tint if impure

Streak: scarlet red

Luster: adamantine in pure crystals to dull in impure masses

Diagnostic Properties: the bright red color and scarlet streak are diagnostic and

distinguish cinnabar from hematite; realgar is red, but has an orange-red streak and a

resinous luster and is very commonly associated with the yellow mineral orpiment




Color/Pleochroism: deep red

Birefringence: 0.35; extremely high; but faded high order colors are masked by mineral

color


Diagnostic Properties: the deep red color and extremely high refractive indices and

relief are characteristic; obtaining good figures is very difficult, so the association with

other low temperature hydrothermal minerals is helpful

Occurrences/Associations: Cinnabar forms primarily in low-temperature hydrothermal veins,

replacement deposits or hot springs associated with volcanism or shallow intrusions. In these

deposits, cinnabar is commonly associated with stibnite, marcasite, realgar, orpiment, pyrite,

opal, chalcedony, calcite, dolomite, fluorite and barite.

Uses: Cinnabar is the principal ore of mercury (Hg) used in electrical apparatuses such as

batteries, switches and mercury-vapor and fluorescent lights, including low-energy light bulbs.

Mercury is also used in thermometers, dental amalgams, as a preservative in many vaccines, in

eyeliners such as mascara and in the production of chlorine for killing microorganisms in water

supplies and swimming pools.

CLAY MINERALS: see CHLORITE, ILLITE, KAOLINITE, and SMECTITE

CLINOZOISITE: see EPIDOTE

COBALTITE:

Composition: (Co,Fe)AsS; a sulfarsenide mineral; isostructural with pyrite

Crystal System: Orthorhombic (mm2): a = 5.58Ǻ, b = 5.58Ǻ, c = 5.58Ǻ; α = 90o, β = 90

o, γ =

90o; so appears isometric (is pseudoisometric)

Crystal Habit: scarce individual crystals are equant, pseudoisometric cubes or pyritohedra;

much more commonly occurs in finely crystalline massive aggregates or coarser granular

aggregates


Hardness: 5½; hard


Cleavage/Fracture: 3 orientations of pseudocubic cleavages at 90o;

Diaphaneity: opaque

Colors: silver white

Streak: gray black

Luster: metallic

Other: commonly alters to pink arsenate mineral erythrite

Diagnostic Properties: can be distinguished from pyrite by its silver white color, its

tendency to alter to pink erythrite and, if discernable, its perfect pseudocubic cleavage

Microscopic Properties: opaque; pleochroic white with pink or violet tint

Occurrences/Associations: Most cobaltite occurs in high temperature hypothermal vein

deposits where it is associated with skutterudite and other cobalt and nickel-bearing minerals.

Uses: Cobaltite is the major ore of cobalt (Co) which is used to harden steel for use in high-speed

tools and to produce permanent magnets. It is also used to produce blue pigments for glassware

and ceramics.

COESITE: see QUARTZ (Silica Group)

COLEMANITE:

Composition: Ca2B6O11•5H2O; a borate mineral


o, γ =

90o

Crystal Habit: typically occurs stubby prismatic crystals that occur in granular aggregates and

cleavable masses


Hardness: 4 – 4½

Specific Gravity: 2.42; low to moderate

Cleavage/Fracture: one orientation; pinacoidal {010}; perfect; there is a second much

less discernible orientation at 90o; pinacoidal {100}; fair


Colors: colorless to white; may have a gray to pale yellow tint

Streak: white


Diagnostic Characteristics: its single perfect cleavage distinguishes it from kernite and

its higher hardness distinguishes it from borax and ulexite which are both soft





Birefringence: 0.028; high; middle second to lower third order colors

Twinning: none

Optic Sign: Biaxial (+); 2Vz = 56o; moderate

Diagnostic Properties: a relatively rare mineral, colemanite can be recognized by the

combination of perfect cleavage, moderate positive relief, high birefringence, biaxial (+)

optics with a moderate 2V, and its association with other continental evaporite minerals

Occurrences/Associations: Most colemanite forms by the diagenetic alteration of primary

borate minerals such as borax and ulexite that formed by evaporation in enclosed, saline lake in

warm, arid climates and by soil water evaporation under similar conditions. Commonly

associated minerals include borax, kernite, ulexite, gypsum, anhydrite, halite and a host of rare

borate minerals.

Uses: The most important use of colemanite is in the production of borax which is in the

manufacture of glass fibers for use in insulation and textiles. Borax is also used in detergent

soaps and as a flux in metallurgical processes such as welding and smelting. One isotope of

elemental boron, obtained from colemanite, is used in the shields of atomic reactors and

elemental boron is also used in fuels for rockets and automobiles and in the high strength plastics

used in aircraft manufacturing.

COPPER:

Composition: Cu; a native element (native metal); may contain some gold (Au) or silver (Ag)

Crystal System: Isometric (4/mbar32/m): a = 3.62Ǻ, b = 3.62Ǻ, c = 3.62Ǻ; α = 90o, β = 90

o , γ =

90o

Crystal Habit: rare, equant, malformed octahedral {111} or dodecahedral {011} crystals; more

typically filiform or scaly; common in dendritic to arborescent or massive aggregates


Hardness: 2½-3; soft


Cleavage/Fracture: no cleavage; hackly fracture

Diaphaneity: opaque

Colors: rose red to copper red

Streak: rose red copper red

Luster: Metallic

Other: Typically alters to reddish copper oxide (cuprite), green or blue copper carbonate

(malachite and azurite) or blue-green copper sulfate.

Diagnostic Properties: Copper’s copper red color, extreme malleability and ductility and

high specific gravity are distinctive, in combination with its alteration products.

Microscopic Properties: Opaque; rose red color and copper red tarnish in reflected light

Occurrences/Associations: Copper typically occurs with zeolite minerals, prehnite, calcite and

epidote in association with mafic volcanic and shallow plutonic rocks such as basalt where it

forms by the reaction of hydrothermal fluids (zeolite facies metamorphism) with oxide minerals

in the basalt. It typically occurs in veins and cavities in the basalts and in pore spaces in adjacent

sedimentary rocks. It also occurs with cuprite, malachite and azurite in the oxidized portion of

hydrothermal veins that contained copper sulfide minerals such as chalcopyrite and bornite.

Uses: An ore of copper used primarily in electrical wiring and in the manufacture of alloys such

as brass (with zinc) and bronze (with tin and zinc) used for various implements. It is also used

by some people for bracelets that are said to cure arthritis and other chronic inflammatory

diseases.

CORDIERITE:

Composition: (Mg,Fe)2Al3(AlSi5O18)•nH2O; a cyclosilicate mineral; iron (Fe+3

) may substitute

for aluminum (Al+3

) and water (H2O), in various amounts, and some carbon dioxide (CO2)

sodium (Na+1


) commonly occur in the ring chambers.


90o, γ = 90

o

Crystal Habit: rare short, psuedohexagonal prisms; more commonly as disseminated anhedral

crystals; less commonly in massive aggregates




Cleavage/Fracture: none discernable; fair to poor cleavages only

Diaphaneity: quite translucent to somewhat translucent; rarely transparent

Colors: typically bluish gray; also gray, dark gray or blue

Streak: white

Luster: vitreous

Twinning: can be complex; simple, lamellar, sector and cyclic twins occur

Diagnostic Properties: The bluish-gray color distinguishes most cordierite from quartz,

which it otherwise resembles; cordierite is softer than corundum which can have similar

colors



Relief: low, negative and/or positive

Color/Pleochroism: colorless; less commonly pale bluish in iron-rich varieties; not

pleochroic

Birefringence: 0.005 – 0.017; low to low-moderate (in iron-rich varieties); typically 1st

order grays and white; maximum first order red colors

Optic Sign: Biaxial (–) mostly; can be (+); 2Vx = 40 – 90o for (–); 2Vz = 74 – 90

o for (+)

Diagnostic Properties: colorless varieties, with their low relief, lowish birefringence

and lack of good cleavage resemble quartz and nepheline, but both of those minerals

are uniaxial, whereas cordierite is biaxial with a substantial 2V and neither quartz nor

nepheline display twinning; orthoclase is biaxial (–), but displays good right angle

cleavage

Occurrences/Associations: Cordierite most commonly occurs in medium to high-grade pelitic

rocks. It is abundant as porphyroblasts generated by contact metamorphism in hornblende

hornfels or pyroxene hornfels facies rocks and in regional metamorphic rocks of the amphibolite

and granulite facies where is associated with sillimanite, kyanite, staurolite, garnet, spinel,

muscovite and biotite. It also occurs in some mafic metamorphic rocks such as amphibolite

where it is associated with amphibole, plagioclase and garnet. Cordierite is less common in

igneous rocks where it occurs in granitoids and pegmatite.

Uses: Cordierite is not an economically important mineral, except as the rare transparent gem

variety of cordierite called iolite.

CORUNDUM:

Composition: Al2O3; an oxide mineral; chromium (Cr+3

), iron (Fe+3

and Fe2+

) and titanium

(Ti+4

), paired with Fe2+

, substitute for aluminum (Al+3

) and have a major effect on color; gem

varieties include ruby (red due to Cr+3

) and sapphire (blue due to Ti+4

and Fe2+

)

Crystal System: Hexagonal: Trigonal (bar32/m): a1 = 4.75Ǻ, a2 = 4.75Ǻ, c = 12.98Ǻ; α =

120o, β = 90

o, γ = 120

o

Crystal Habit: columnar prisms with hexagonal outlines common; less commonly as tabular

crystals with hexagonal outlines; divergent crystal aggregates; massive to granular aggregates

and disseminated crystals


Hardness: 9; very hard

Specific Gravity: 4.02; high or moderate-high

Cleavage/Fracture: no cleavage, but prominent parting (basal and prismatic) occurs

Diaphaneity: transparent to translucent

Colors: typically gray to gray blue or gray red; also white, yellow-green, red (ruby), or

blue (sapphire)

Streak: white, but hardness makes it difficult to obtain streak

Luster: vitreous to adamantine; subvitreous when altered

Other: commonly alters to muscovite/sericite; also to clay minerals and bauxite;

oriented inclusions in gem varieties may cause asterism, as in star sapphires

Diagnostic Properties: the extreme hardness (if not altered) and the hexagonal cross-

sections of crystals are diagnostic; the parting, high luster and specific gravity are helpful


Refractive Indices: nω = 1.772 – 1.776; nε = 1.764 – 1.768; higher with increasing

chromium and iron


Color/Pleochroism: colorless or pale shades of blue, blue-gray, blue-green, gray or light

red; not pleochroic in thin-section

Birefringence: 0.008; low; maximum colors are first order yellows; higher orders

sometimes observed because hard corundum is often thicker than the standard in thin

sections, increasing retardation

Twinning: lamellar twins are common

Optic Sign: Uniaxial (–); some crystals have biaxial characteristics with substantial 2V;

Other: longitudinal sections are length fast; crystals commonly zoned or color banded;

inclusions of hematite, rutile and spinel are common; parting can resemble cleavage, but

with larger spacing

Diagnostic Properties: the combination of very high relief, low birefringence, and

hexagonal crystals is characteristic; lamellar twinning and/or zoning help to confirm

identification as does uniaxial (–) figure, if it can be obtained

Occurrences/Associations: Corundum commonly occurs in peraluminous, silica undersaturated

alkali igneous rocks such as alkali syenite, foid syenite, alkali pegmatite and lamprophyre. It is

also common in many medium to high grade metamorphic rocks including pelitic schist and

gneiss, hornfels, marble and skarns where it is associated with andalusite, sillimanite, cordierite,

scapolite, muscovite, calcite and dolomite. An unusual occurrence is in emery deposits which

are believed to have formed by the metamorphism of bauxite-rich soils. Because it is very hard

and chemically resistant, corundum is also concentrated in detrital sediments. That is where

many of the gemstones are collected.

Uses: The most valuable forms of corundum are the prized gemstones ruby and sapphire. The

highest quality, natural rubies are more valuable even than diamonds, and considerably rarer.

Color-enhanced natural and artificial rubies and sapphires are produced, but are considerably less

valuable. Due to its great hardness, corundum, especially in the form of emery, is mined for use

as an abrasive agent for sand paper, grinding wheels, polishing materials and cutting tools.

COVELLITE:

Composition: CuS; a sulfide mineral

Crystal System: Hexagonal (6/m2/m2/m): a1 = 3.79Ǻ, a2 = 3.79Ǻ, c = 16.34Ǻ; α = 120o, β =

90o, γ = 120

o

Crystal Habit: scarce individual crystals typically tabular, with hexagonal outlines; more

commonly as massive or foliated aggregates or as disseminated crystals


Hardness: 1.5 – 2; soft


Cleavage/Fracture: one perfect basal pinacoidal {0001} cleavage

Diaphaneity: opaque

Colors: richly bluish (indigo) gray; may be iridescent

Streak: gray black

Luster: Metallic

Other: often altered to copper carbonates such as malachite and azurite and oxides such

as cuprite, as well as to supergene minerals such as chalcocite and rarer copper sulfides

Diagnostic Properties: The richly bluish gray color and perfect cleavage are distinctive

Microscopic Properties: opaque; pleochroic indigo blue to bluish white in reflected light

Occurrences/Associations: Covellite commonly occurs in the zone of supergene enrichment

where groundwater concentrates metal from minerals in hydrothermal vein and replacement

deposits. In such deposits, it is commonly associated with chalcocite and several rarer copper

and silver sulfide minerals. Covellite also occurs in hydrothermal vein and replacement deposits

with bornite, chalcopyrite, pyrite and tetrahedrite.

Uses: Covellite is fairly scarce and so is a minor ore of copper. Copper used primarily in

electrical wiring and switches and in the manufacture of alloys such as brass (with zinc), bronze

(with tin and zinc) and German silver (with zinc and nickel) used for various implements.

CRISTOBALITE:

Composition: SiO2; a tectosilicate mineral; a silica group mineral; inclusion of small amounts of

sodium (Na+1

), potassium (K+1

) and (Ca+2

) in the crystal structure is electrically balanced by the

substitution of aluminum (Al+3

) for silicon (Si+4

); occurs in two polytypes, both stable at

relatively low pressure: tetragonal α – cristobalite (low tridymite) and isometric β – tridymite

(high tridymite); α – cristobalite (low cristobalite) is the stable form in normal temperature

environments; high temperature β – cristobalite (high cristobalite) inverts readily, through non-

displacive transformation, into α – cristobalite (low cristobalite) on cooling; the characteristics

of α – cristobalite (low cristobalite) are therefore cited below; cristobalite is polymorphic with

other silica group minerals including quartz and tridymite; rarer polymorphs include the high

pressure silica minerals coesite and stichovite which occur in meteorite impactites and likely in

the deep mantle

Crystal System: Tetragonal (422): a1 = 4.97Å, a2 = 4.97Å, c = 6.93Å; α = 90o, β = 90

o, γ = 90

o;

crystals are commonly pseudomorphs after isometric β – cristobalite (high cristobalite), thus

pseudo-isometric

Crystal Habit: typically occurs as equant octahedral, rarely cubes; also as dendritic to skeletal

crystals in radiating to stellated (spheroidal) aggregates; also capillary crystals in fibrous

aggregates


Hardness: 6 ½ - 7; hard

Specific Gravity: 2.3 – 2.4; low to low-moderate


Diaphaneity: transparent to moderately translucent

Colors: colorless to white or pale yellow

Streak: colorless

Luster: vitreous

Other: α–cristobalite (low cristobalite) pseudomorphs after isometric β–cristobalite (high

cristobalite are the rule; sometimes α–quartz pseudomorphs occur, although the

inversion is displacive, so the kinetics are slow

Diagnostic Properties: cristobalite is difficult to distinguish from other macrocrystalline

varieties of silica such as quartz, without optical or XRD methods; the octahedral crystals

are suggestive, the isometric nature of α – cristobalite (low cristobalite) is distinctive and

helps to distinguish it from tridymite


Refractive Indices: nα = 1.486 – 1.488; nε = 1.482 – 1.484



Birefringence: 0.002 – 0.004; quite low; maximum first order grays; can appear nearly

isotropic

Twinning: multiple twins common, either in lamellar or cross-hatch patterns


Other: crystals may be length slow or length fast

Diagnostic Properties: euhedral crystals can be distinguished by their octahedral or

cubic form, moderate negative relief, low birefringence and uniaxial (–) optics; the finely

crystalline material resembles some tridymite and chalcedony and requires advanced

techniques such as XRD for identification

Occurrences/Associations: Cristobalite is a relatively scarce mineral that occurs primarily in

silica-oversaturated, felsic volcanic and hypabyssal rocks, especially rhyolite, rhyodacite and

dacite as phenocrysts, as part of the groundmass or in spherulites (with sanidine) formed by the

divitrification of glass. It can form pseudomorphs after tridymite. It also lines vesicles in felsic

and intermediate volcanic and hypabyssal rocks. Cristobalite is also reported from stony

meteorites and as a fibrous mineral in some serpentenite.

Use: Cristobalite has no economic value.

CUMMINGTONITE:

Composition: (Mg,Fe)2(Mg,Fe)5Si8O22(OH)2; double-chain inosilicate; an amphibole; forms a

solid solution series with grunerite [(Mg,Fe)2(Mg,Fe)5Si8O22(OH)2] which contains smaller

amounts (< 30%) of magnesium (Mg+2

) and larger amounts ( >70%) of iron (Fe+2

);

cummingtonite is related to anthophyllite, but larger amounts of iron (Fe+2

) in cummingtonite

cause it to crystallize in the monoclinic, rather than the orthorhombic, system

Crystal System: Monoclinic (2/m): a = 9.52 – 9.54Å, b = 18.14 – 18,26Å, c = 5.31 – 5.32Å; α

= 90o, β = 102

o, γ = 90

o

Crystal Habit: slender, bladed prismatic; crudely with diamond-shaped cross-sections; in

foliated-lamellar to massive aggregates; also as acicular crystals in fibrous to radiated aggregates


Hardness: 5 - 6

Specific Gravity: 3.1 – 3.4; moderate; grunerite is higher


(56o and 124

o); very good

Diaphaneity: somewhat to slightly translucent

Colors: typically medium to dark brown or dark greenish brown; also gray-brown and

tan

Streak: white

Luster: vitreous to silky; also subvitreous

Diagnostic Properties: Distinguished from pyroxenes which possess near right angle

cleavage by its cleavages not at right angles (when it is discernable); brown varieties

resemble anthophyllite, but are generally darker; green varieties resemble hornblende, but

are more typically occur in radiated to fibrous aggregates



grunerite has higher indices

Relief: moderate to high; positive

Color/Pleochroism: colorless to grayish brown; may be weakly pleochroic with

colorless, pale yellow, pale brown, pale greenish yellow, even pale violet

Birefringence: 0.20 – 0.34; moderate; maximum colors generally low to high second

order

Twinning: common; simple pairs or lamellar

Optic Sign: Biaxial (+), 2Vz = 65-90o; high to very high; grunerite is negative

Other: cross-sections display symmetrical extinction; longitudinal sections are length-

slow and display relatively small extinction angles (14 – 21o) relative to cleavage;

commonly alters to chlorite, talc and/or serpentine

Diagnostic Properties: distinguished from pyroxenes by 2 sets of cleavage not at right

angles; cleavage may not be visible in fibrous aggregates; the pleochroism and pale

colors in shades of pale shades of brown, yellow, green and violet is somewhat similar

to anthophyllite, but anthophyllite has parallel extinction because it is orthorhombic; most

other monoclinic amphiboles are biaxial (-) and most possess different colors

Occurrences/Associations: Cummingtonite is common in medium- to high-grade mafic

metamorphic rocks such as gneiss and granulite, where it is associated with cordierite,

plagioclase, garnet and hornblende. It also occurs in some magnesian (ultramafic) metamorphic

rocks.

Uses: Cummingtonite has been used as a substitute for chrysotile asbestos which was long used

in fireproofing and insulation. However, it turns out to have similar negative health implications.

CUPRITE:

Composition: Cu2O; an oxide mineral (X2O group)

Crystal System: Isometric (4/mbar32/m): a1 = 4.27Ǻ, a2 = 4.27Ǻ, a3 = 4.27Ǻ; α = 90o, β = 90

o,

γ = 90o

Crystal Habit: commonly as equant octahedral {111} crystals; also cubic {001} and

dodecahedral {110} crystals, often combined with octahedra; mostly in massive or granular

aggregates




Cleavage/Fracture: generally not discernible; 4 orientations; octahedral {111}; fair

Diaphaneity: moderately to barely translucent

Colors: ruby red to almost black

Streak: deep brownish red

Luster: adamantine to submetallic

Other: alters to malachite, azurite, native copper and other copper minerals

Diagnostic Properties: The deep red (to almost black) color and deep brownish red

streak are characteristic; distinguished from specularite hematite by cuprite’s inferior

hardness, darker streak and isometric crystals and from cinnabar by the latter’s scarlet red

streak and inferior hardness


Refractive Indices: n = 2.849


Color/Pleochroism: red to orange yellow; not pleochroic since isotropic

Birefringence: none; isotropic

Twinning: none


Diagnostic Properties: the combination of extremely high refractive indices and relief,

its isotropic nature and its red to orange-yellow color are diagnostic; cinnabar is

somewhat similar, but has a deeper red color, is anisotropic and has a rather different

association, forming in low-temperature hydrothermal conditions, rather than in the

oxidized zone of such deposits as for cuprite

Occurrences/Associations: Most cuprite forms in the oxidized zone of hydrothermal vein and

replacement deposits that contain copper-bearing sulfide minerals such as chalcopyrite and

bornite. These are oxidized by descending, acidic (CO2-rich) meteoric water to produce cuprite

which is typically associated with malachite, azurite, native copper, limonite (goethite),

smithsonite and anglesite, among others.

Uses: Cuprite is a minor ore of copper which used primarily in electrical wiring and switches and

in the manufacture of alloys such as brass (with zinc), bronze (with tin and zinc) and German

silver (with zinc and nickel) used for various implements.

DIAMOND:

Composition: C; polymorphic with graphite which is stable at lower pressures

Crystal System: Isometric (4/m/2/m2/m): a = 3.56Ǻ, b = 3.56Ǻ, c = 3.56Ǻ; α = 90o, β = 90

o,

γ = 90o

Crystal Habit: equant, mostly octahedral {111} crystals, but dodecahedral {011} and cubic

{001} forms occur; may occur in small, polycrystalline aggregates


Hardness: 10; the hardest known mineral

Specific Gravity: 3.5; moderate to high

Cleavage/Fracture: 4 sets not at 90o; octahedral {111}; perfect

Diaphaneity: transparent

Colors: variable; colorless, white, pale yellow, blue; less commonly orange, pink,

Streak: white, but too hard to produce a conventional streak

Luster: adamantine when cut; greasy in uncut stones

Diagnostic Properties: The octahedral crystal form, extreme hardness and luster are

characteristic of diamond



Relief: very high-extreme

Color/Pleochroism: colorless, but rare and can’t be ground in thin-section, unless

diamond used as the abrasive

Optics: isotropic

Diagnostic Properties: rarely seen in thin-section, unless diamond is used as the

abrasive; the extreme refractive index and relief, isotropic optics, lack of color in thin

section, octahedral crystals and four sets of cleavage would be diagnostic

Occurrences/Associations: Natural diamonds are high pressure minerals that occur primarily in

ultramafic rocks called kimberlites that typically occur in brecciated pipes called diatreme and in

detrital alluvial deposits derived from them. Diamond-bearing kimberlites typically contain

fragments of peridotite rich in olivine, pyroxene (especially chrome diopside), garnet, spinels

and phlogopite. These and the diamonds are thought to have formed at depth in the mantle and

have been brought near the surface during explosive eruptions of gas-rich kimberlite magmas.

The precise mechanism of emplacement remains controversial. Small, very rare diamonds occur

in ultrahigh pressure (UHP) metamorphic rocks and graphite deposits metamorphosed during

meteorite impacts. Most industrial diamonds are produced synthetically.

Uses: Diamond is a very important gemstone; better marketed than any other. Low quality and

synthetic diamonds are widely used in cutting tools such as rock saws and drills and in abrasives

for polishing surfaces and for sharpening instruments

DIOPSIDE:

Composition: CaMgSi2O6; single-chain inosilicate; a clinopyroxene; forms a complete solid

solution series with hedenbergite (CaFeSi2O6); one classification scheme recognizes diopside

(> Di90), salite (Di50–90), ferrosilite (Di10–50) and hedenbergite (< Di10); these minerals also

display extensive solid solution with augite [(Ca,Na)(Mg,Fe,Fe,Al,Ti)Si2O6]


o, γ = 90

o


cross sections; may also show prominent parting; commonly in granular aggregates or coarse

cleavable aggregates or as disseminated crystals





o);

good only; prominent parting in some examples

Diaphaneity: nearly transparent to moderately translucent

Colors: typically white to pale green, apple green or medium green; emerald green in

chromium-rich varieties

Streak: white

Luster: vitreous

Other: commonly alters to chlorite, serpentinite, carbonates and clay minerals

Diagnostic Properties: The pale green to apple green color distinguishes diopside from

all pyroxenes except jadeite; distinguished from forsterite-rich olivine by possessing good

cleavage


Refractive Indices: nα = 1.664 – 1.676; nβ = 1.672 – 1.685; nγ = 1.694 – 1.706; may be

higher in iron-rich diopside


Color/Pleochroism: colorless to very pale green; iron-rich varieties display deeper colors

and may display pleochroism in blue green, brownish green and yellowish green

Birefringence: 0.018 – 0.030; moderate; maximum first order red to second order red

colors


Optic Sign: Biaxial (+); 2Vz = 50 – 60

o; moderate 2V

Other: cross-sections exhibit symmetrical extinction; commonly alters to talc,

serpentine, tremolite or chlorite


cleavage near right angles distinguish diopside from amphiboles; hedenbergite has higher

refractive indices and relief and possesses deeper colors; augite is similar in most respects

and the distinction between the two is difficult; pigeonite has a smaller 2V (< 32o); some

augite possesses a lower 2V, slightly higher extinction angles and augite is more common

in mafic intrusive igneous rocks and much less common in calcareous metamorphic rocks

Occurrences/Associations: Diopside is an abundant mineral in calcareous metamorphic rocks

such as marble, skarn and calcsilicate rock. Such rocks are produced by the contact and/or

regional metamorphism of impure dolostone and limestone under medium- to high-grade

metamorphism in the hornblende hornfels, pyroxene hornfels, amphibolite and even granulite

facies. Diopside is commonly associated with calcite, dolomite, grossular garnet, tremolite,

wollastonite, scapolite, vesuvianite, forsterite-rich olivine and epidote group minerals. Diopside

also occurs in medium-high grade mafic and quartzofeldspathic rocks including amphibolite,

hornblende gneiss and pyroxene granulite. Chrome-rich diopside is common in ultramafic rocks

including peridotite and kimberlite. Diopside also occurs in some lamprophyre dikes and as

phenocrysts in olivine basalt.

Uses: Some diopside, especially chromium-rich diopside, is used as a semi-precious gemstone.

DOLOMITE:

Composition: CaMg(CO3)2; a carbonate; isostructural with ankerite CaFe(CO3)2; a complete

solid solution series exists between these two “double-layer” rhombohedral carbonate minerals.

Crystal System: Hexagonal (Rhombohedral) (bar32/m): a1 = 4.80Ǻ, a2 = 4.80Ǻ, c = 15.98Ǻ; α

= 120o, β = 90

o, γ = 120

o

Crystal Habit: equant rhombohedral {10bar11} crystals with curved faces common; common as

massive microscopic to coarsely crystalline granular and massive aggregates; also occur as ooids


Hardness: 3½


Cleavage/Fracture: 3 cleavages, not 90o (73

o and 107

o); rhombohedral {10bar10};

excellent to perfect

Diaphaneity: translucent

Colors: typically white or gray; often tinged yellow or brown

Streak: white


Other: rhombohedral twins common; ankerite-bearing varieties weather yellow-brown

Diagnostic Properties: similar to calcite, but does not readily effervesce in cold, dilute

HCl unless sample is powdered of acid is heated; tendency for Fe-bearing dolomite to be

yellow-brown is also useful


Refractive Indices: nω = 1.682; nε = 1.500; increasing with Fe content

Relief: variable; from low negative to moderately-high positive; sections in which both

nω and nε are visible produce a “twinkling” effect as the relief changes when stage is

rotated under plane light


Birefringence: 0.180; very high-extreme; so high order, faded colors (“creamy white”)

in most orientations

Twinning: rhombohedral twins are common


Other: extinction is symmetrical with respect to intersecting cleavages; dolomite twins

parallel both the long and short diagonals between cleavage traces



uniaxial (–) optics are characteristic of rhombohedral carbonates such as dolomite;

dolomite is distinguished from calcite only with difficulty; dolomite tends to occur more

commonly as euhedral rhombohedra in sedimentary rocks; is less commonly twinned

than is calcite, possesses twins that parallel both the long and short diagonals of cleavage

traces (calcite twins parallel only the long diagonal) and tends to be stained with iron

oxides more commonly because iron (Fe+2

) substitutes easily for the magnesium (Mg+2

)

in dolomite

Occurrences/Associations: Dolomite is abundant in carbonate sedimentary rocks as the

essential constituent of dolostone. The vast majority of dolostones form by the diagenetic

replacement of limestone by Mg-rich, low sulfate groundwater that converts calcite to dolomite.

Dolomite is also a common mineral in calcareous metamorphic rocks such as dolomitic marble,

calcsilicate rock and skarn where it occurs with diopside, tremolite, phlogopite, calcite and

grossularite garnet. Dolomite (and ankerite) is also a common gangue mineral in hydrothermal

veins and replacement deposits. Rare dolomite (and ankerite) occurs in carbonatite igneous

rocks associated with silica undersaturated rocks such as foid syentite.

Uses: By heating dolomite and driving off carbon dioxide (CO2) both calcium oxide (CaO =

lime) and magnesium oxide (MgO) are produced. Lime is used in cement products, aggregates

such as concrete and to whitewash buildings and line fields. Magnesium oxide is used in the

production of refractory bricks or linings for furnace and industrial oven linings. Crushed

dolomite is used in aggregates such as concrete and asphalt. Dolomite is also used as a

dimension stone for construction. Most magnesium for other uses is currently derived from

seawater or groundwater brines.

DRAVITE: see TOURMALINE

ELBAITE: see TOURMALINE

EMERALD: see BERYL

ENSTATITE: see ORTHOPYROXENE

Stopped here

EPIDOTE (and CLINOZOISITE and PIEMONTITE):

Composition: Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH); sorosilicate; solid solution series with

clinozoisite which contains less iron (Fe) and more aluminum (Al); the transition between the

two is generally taken to be the transition from optically (–) epidote to optically (+) clinozoisite

which typically occurs between 10 – 15% Fe+3

; where a few percent of manganese (Mn+2

)

substitutes, the pink epidote mineral called piedmontite forms; all three may be intergrown with

zoisite

Crystal System: Monoclinic (2/m): a = 8.88 – 8.89Å, b = 5.58 – 5.63Å, c = 10.15 – 10.16; α =

90 o, β = 115

o, γ = 90

o

Crystal Habit: individual crystals range from acicular to bladed and prismatic; radial aggregates

of acicular crystals and divergent aggregates of bladed-prismatic crystals; also as disseminated

crystals and in massive to granular aggregates


Hardness: 6 – 7; hard

Specific Gravity: 3.21 – 3.49; moderate; higher for iron-rich epidote

Cleavage/Fracture: one orientation of basal pinacoidal {001} cleavage; excellent to

perfect

Diaphaneity: somewhat to barely translucent

Colors: epidote is grass green to pistachio green to greenish black; clinozoisite is pale

green to gray; piedmontite is pink to reddish brown

Streak: white

Luster: dull in earthy varieties; vitreous in macro-crystals; silky in radiating to stellate

aggregates

Diagnostic Properties: Epidote is distinguished by its grass to pistachio green color, its

hardness and, where visible, its single cleavage; the radiating aggregates of acicular

crystals are also distinctive; olivine lacks cleavage and is typically a paler shade of green


Refractive Indices: epidote: nα = 1.714 – 1.751; nβ = 1.720 – 1.784; nγ = 1.727 – 1.797;

clinozoisite: nα = 1.703 – 1.716; nβ = 1.707 – 1.723; nγ = 1.709 – 1.731; piedmontite:

nα = 1.725 – 1.756; nβ = 1.730 – 1.789; nγ = 1.750 – 1.832

Relief: moderately high to very high, positive relief; increases with iron content, so

higher for epidote than for clinozoisite

Color/Pleochroism: epidote is weakly pleochroic in pale shades of green, yellow,

yellow green and greenish brown; clinozoisite is colorless; piedmontite is pleochroic

yellow, pink, orange, red, brownish red, purple red and violet

Birefringence: epidote: 0.012 – 0.049; low to high; maximum first order yellow to third

order colors; clinozoisite: 0.006 – 0.015; low; maximum first order gray first order red

colors; piedmontite is 0.028 – 0.082; moderate to very high; maximum second- to

fourth-order colors

Twinning: uncommon, lamellar

Optic Sign: epidote: Biaxial (–); 2Vx = 64 – 90o; high to very high 2V; clinozoisite:

Biaxial (+); 2Vz = 15 – 90o; low to very high 2V; piedmontite: Biaxial (+); 2Vz = 50 –

86o

Other: epidote commonly exhibits a marked mottling to its birefringence, displays

abnormal bluish to greenish-yellow interference colors similar to clinozoisite and may be

zoned; clinozoisite commonly displays abnormal bluish to greenish yellow interference

colors like epidote and may also be zoned; there is an inverse relationship between 2V

and extinction angle; piedmontite is commonly zoned; all three minerals may occur

together in zoned crystals

Diagnostic Properties: epidote is distinguished from clinozoite by its biaxial (–) optics,

its yellow to green color and its generally higher birefringence; clinozoisite is

distinguished from epidote by its biaxial (+) optics, generally lower birefringence, lack of

color and common anomalous interference colors; vesuvianite superficially resembles

clinozoisite, but is generally uniaxial, lacks good cleavage and crystallizes in the

tetragonal system; piedmontite is distinguished from both epidote and clinozoisite by its

deeper colors, especially the violet tints, its generally higher birefringence and refractive

indices and relief; piedmontite resembles thulite, but the latter is orthorhombic, thus

possesses parallel extinction

Occurrences/Associations: Epidote occurs widely. It is an important component of low-grade

to lower medium-grade quartzofeldspathic, calcareous and especially mafic metamorphic rocks

formed in the albite-epidote hornfels, greenschist and epidote amphibolite facies where it is

associated with albitic plagioclase, chlorite, actinolite, hornblende, quartz, calcite, vesuvianite,

garnet, and diopside. Clinozoisite occurs under similar conditions in quartzofeldspathic,

calcareous and especially pelitic rocks. Epidote and clinozoisite are common as accessory

minerals in granitoids. They are also produced by hydrothermal alteration in fault zones and,

quite commonly, by the alteration of plagioclase (sausseritization) in granitoids. Both epidote

and clinozoisite are fairly resistant to decomposition during weathering and so occur in detrital

sediments derived for source rocks similar to those described above. Piedmontite is a rarer

mineral that occurs in metamorphic rocks, especially manganese rich ore deposits of

metamorphic or hydrothermal origin.

Uses: Epidote and clinozoisite are not economically valuable minerals. They are occasionally

cut for use as semiprecious gemstones.

FAYALITE: see OLIVINE

FLINT: see CHERT

FORSTERITE: see OLIVINE

FLUORITE:

Composition: CaF2; a halide mineral; some rare earth elements such as cerium (Ce+2

) and

yttrium (Y+2

) may substitute in limited amounts for calcium


o,

γ = 90o

Crystal Habit: Equant cubic {001} crystals; less commonly octahedral {111}; massive to

granular aggregates


Hardness: 4; moderate


Cleavage/Fracture: 4 orientations, not 90o; octahedral {111}; perfect; triangular faces

bounded by three of the cleavages with the face the fourth cleavage are common

Diaphaneity: quite translucent to transparent

Colors: variable; colorless, purple, yellow, blue, green all fairly common

Streak: white

Luster: vitreous

Diagnostic Properties: The moderately hard cubic crystals and the octahedral cleavage

with triangular faces, in combination with its translucency and vitreous luster are

distinctive.


Refractive Indices: n = 1.43-1.44; higher where rare earth element content is high

Relief: moderate; negative; one of the few common minerals with moderate, negative

relief

Color/Pleochroism: generally colorless or very pale hues in blues and purples; not

pleochroic; pale colors may be “patchy” or mottled



Other: perfect cleavages typically seen as 2 sets that intersect at 70o and 110

o or as three

sets that intersect at 60o and 120

o

Diagnostic Properties: the moderate negative relief, isotropic optics, pale patchy colors,

if any and the perfect cleavage patterns are diagnostic

Occurrences/Associations: Fluorite is a common mineral in low-medium temperature

hydrothermal deposits where it occurs in veins and replacement deposits with sphalerite, galena,

pyrite, calcite, barite and celestite. Fluorite also occurs as a minor accessory mineral in alkali-

rich igneous rocks such as granite, syenite and alkali granite pegmatite, especially in cassiterite-

bearing S-type granitoids. Fluorite is disseminated in some sedimentary rocks, especially

carbonate rocks such as limestone and dolostone.

Uses: Fluorite is the major source of fluorine used in various industrial processes; as a flux in

productions and in the manufacture of fluorocarbons for use in air conditioners and refrigerators.

Fluoride compounds are used in the fluoridation of drinking water, toothpastes and mouthwashes

designed to reduce dental caries. Fluorite is also used in the preparation of hydrofluoric acid

(HFl).

GALENA:

Composition: PbS; a sulfide mineral; isostructural with halite


o, γ

= 90o

Crystal Habit: typically as equant cubes {001}, less commonly octahedra {111}, sometimes in

combination; common in massive, cleavable aggregates, but also as disseminated crystals


Hardness: 2½; soft


Cleavage/Fracture: 3 orientations at 90o; cubic {001}; excellent to perfect

Diaphaneity: opaque

Colors: dark gray (lead gray)

Streak: dark gray (lead gray)

Luster: metallic

Other: commonly alters to cerrusite and anglesite

Diagnostic Properties: the combination of lead gray color and streak, metallic luster,

cubic cleavage, very high specific gravity and soft hardness distinguish galena from

all other minerals (especially other gray sulfides)

Microscopic Properties: Opaque; white in reflected light, sometimes with a pinkish hue

Occurrences/Associations: Galena is widespread in hydrothermal veins and replacement

deposits produced under a variety of temperatures that range from high temperature hypothermal

through low temperature epithermal and telethermal conditions. Commonly associated minerals

include sphalerite, pyrite, marcasite, chalcopyrite and a host of other sulfides, as well as calcite,

quartz, fluorite and barite.

Uses: Galena is the major ore of lead used in the manufacture of lead-acid batteries such as those

used in automobiles and tractors. Combined with tin and antimony, lead is used to produce

electrical solders. Lead is also used as a shield in nuclear reactors. Formerly, lead was widely

used in paints, pipes and pottery glazes and to boost the octane rating of gasoline, but the toxic

effects of lead poisoning have led to a severe restriction of these uses. Some galena contains

sufficient silver (Ag) in solid solution to be a significant ore of silver.

GARNET: (GARNET GROUP)

Composition: X3Y2(SiO4)3; nesosilicate (orthosilicate) minerals: the garnet group includes the

varieties almandine [Fe3Al2(SiO4)3], andradite [Ca3Fe2(SiO4)3, grossular(ite) [CaAl2(SiO4)3],

pyrope [Mg3Al2(SiO4)3], spessartine [Mn3Al2(SiO4)3] and uvarovite [Fe3Cr2(SiO4)3] between

which extensive solid solution exists, especially between almandine, spessartine and pyrope

(Fe+2

, Mn+2

and Mg+2

) and between andradite and grossularite (Fe+3

and Al+3

); almandine and

spessartine are sometimes referred to as almandite and spessartite respectively

Crystal System: Isometric (4/mbar32/m): a1 = 11.46Å, a2 = 11.46Å, a3 = 11.46Å; α = 90o, β =

90o, γ = 90

o

Crystal Habit: common as equant dodecahedra {011} and/or trapezohedra {hhl}; can resemble

small soccer balls with diamond-shaped or trapezoidal faces; as disseminated crystals or as

massive to granular aggregates


Hardness: 6½ - 7½; hard to very hard

Specific Gravity: 3.5 – 4.3; moderate plus to high

Cleavage/Fracture: none; conchoidal fracture; prominent parting in some specimens


Colors: extremely variable; almandine and spessartine most often wine red, like

raspberry jam or reddish brown, but also brown; pyrope is typically pink, purple, red or

nearly black; grossular is often green, but also white, yellow, cinnamon brown or pale

red; andradite is typically yellow to green, but also brown or black; uvarovite is

emerald green

Streak: white


Other: almandine and andradite commonly alter to chlorite, epidote and/or iron oxides;

pyrope commonly alters to serpentine and talc

Diagnostic Properties: the equant, dodecahedral to trapezohedral crystals are

characteristic; the hardness, lack of cleavage, conchoidal fracture and some colors

(especially wine red to red brown) are also distinctive


Refractive Indices: 1.720 – 1.890; lowest for pyrope and grossularite (1.720 – 1.770,

highest for andradite and uvarovite (1.850 – 1.895) and intermediate for almandine and

spessartine (1.770 – 1.820)

Relief: high to very high, positive

Color/Pleochroism: colorless to quite pale rose/pink most commonly, but also pale

shades of other hand sample colors; not pleochroic; in thin-section, spessartine and

grossularite are colorless, almandine is colorless to pale rose, pyrope is pale pink,

andradite is pale brown and uvarovite is pale green

Birefringence: none; isometric

Optic Sign: none, isometric

Other: commonly zoned; some garnets (especially spessartine and grossularite) may be

slightly anisotropic and display birefringence with low first order colors

Diagnostic Properties: the high relief, isotropic optics, equant six-sided cross sections

(of dodecahedra) and eight-sided cross sections (of trapezohedra) and lack of cleavage

are characteristic of all garnet; given the amount of substitution solid solution between

garnets, a chemical analyses are helpful in determining the exact composition of any

garnet; educated approximations may be made on the basis of color, refractive index

and occurrences/associations (see above and below).

Occurrences/Associations: Garnet group minerals of various compositions are widely

distributed in metamorphic rocks. The most abundant is almandine-rich garnet which is

abundant in pelitic rocks and common in mafic/basic rocks formed over a wide range of

conditions, including regional metamorphic rocks of the greenschist, amphibolite and granulite

facies and contact metamorphic rocks of the albite-epidote hornfels, hornblende hornfels and

pyroxene hornfels facies. Grossular-rich and andradite-rich garnet occurs primarily in

calcareous metamorphic rocks such as marble, calcsilicate rock and skarn, again developed over

a wide range of contact and regional metamorphic conditions. Scarcer spessartine-rich garnet

occurs in similar circumstances, but only where large amounts of manganese are available.

Pyrope-rich garnet and rare uvarovite-rich garnet occur chiefly in ultramafic/ultrabasic igneous

and metamorphic rocks such as peridotite, serpentinite and kimberlite. Garnet in eclogite is

typically intermediate between pyrope and almandine. Garnet, typically intermediate between

almandine and spessartine, also occurs as an accessory mineral in igneous rocks, especially in

pegmatite and some peraluminous granitoids and syenite.

Uses: Garnet is mined extensively for use as an abrasive, especially for use in sandpaper and

emery sticks for fingernails. It is also utilized in filters used in water purification systems.

Several varieties of garnet are utilized as semi-precious gemstones.

GIBBSITE: see BAUXITE

GLAUCONITE:

Composition: (K,Na,Ca)0.5-1.0(Fe,Mg,FeAl)2(Si,Al)4O10(OH)2•nH2O; a phyllosilicate mineral;

T-O-T structure; a clay mineral; frequently a mixed layer clay that contains some expandable

layers


o, γ =

90o

Crystal Habit: scaly-flaky to tabular crystals; typically occurs as small (< 2.0mm) granules or

pellets that are aggregates of numerous tiny crystals


Hardness: 2; soft

Specific Gravity: 2.4 – 2.9; light to moderate

Cleavage/Fracture: one orientation of basal pinacoidal {001} cleavage; perfect, but

rarely discernible in aggregates


Colors: pleochroic; mostly in shades of green; green, yellow, yellow-green, olive green,

blue green common

Streak: light green to greenish gray

Luster: subvitreous to dull/earthy

Other: alters to limonite, giving it a yellow- to red-brown color

Diagnostic Properties: The green color, low hardness and pelletal aggregate grains are

distinctive as is the occurrence in marine sediments and sedimentary rocks; however

some green pellets in sedimentary rocks are chlorite




Color/Pleochroism: pleochroic in shades of green, yellow green and blue green; yellow-

brown where altered to limonite

Birefringence: 0.014 – 0.032; low to moderate; second order colors are typically masked

by the color, making it appear to have lower birefringence

Twinning: none

Optic Sign: Biaxial (–); 2Vx = 0 – 20o; (usually >10

o); very low 2V; good figures

difficult to obtain

Other: cleavage traces, if visible, are length slow

Diagnostic Properties: the combination of pelletal-granular shape, moderate relief,

microcrystalline structure, greenish color and occurrence in sedimentary rocks are

distinctive; some mixed-layer clay pellets that contain chlorite may appear similar and are

distinguishable only by using XRD or other advanced analytical techniques

Occurrences/Associations: Glauconite occurs chiefly in marine sediments and sedimentary

rocks, where it often forms by the replacement of organic-rich pellets in slightly reducing

environments. It may be the principal constituent of “greensands” and occurs widely as

disseminated pellets in limestone and argillaceous limestone.

Uses: Glauconite is used chiefly as a soil additive which provides potassium for plant growth

and loosens clayey soils, while aiding water retention. It is also used in water treatment facilities

to remove iron, manganese and hydrogen sulfide from solution.

GLAUCOPHANE:

Composition: Na2Mg3Al2Si8O22(OH)2; double-chain inosilicate; sodic amphibole group

mineral; glaucophane forms a complete solid solution series with riebeckite

Na2Fe3Fe2Si8O22(OH)2 in which ferrous iron (Fe+2

) substitutes for magnesium (Mg+2

) and ferric

iron (Fe+3

) substitutes for aluminum (Al+3

); glaucophane in which >50% ferrous iron (Fe+2

)

substitutes for magnesium (Mg+2

) is called ferroglaucophane and reibeckite in which >50%

magnesium (Mg+2

) substitutes for ferrous iron (Fe+2

) is called magnesioriebeckite; intermediate

compositions with 30 – 70% each of the reibeckite and glaucophane end members are called

crossite [Na2(Mg,Fe)3(Al,Fe)2Si8O22(OH)2]; glaucophane also forms a solid solution series with

tremolite-actinolite [Ca2(Mg,Fe)5Si8O22(OH)2]

Crystal System: Monoclinic: (2/m): a = 9.5 – 9.6Å, b = 17.7 – 17.8Å, c = 5.3Å; α = 90o, β =

104o, γ = 90

o

Crystal Habit: typically as thin prismatic to bladed crystals, with six-sided, diamond shaped

cross sections; as massive to granular or foliated aggregates and as disseminated crystals; less

commonly acicular; in fibrous or massive aggregates





(56o and 124

o); very good; splintery; not discernible in finely crystalline aggregates

Diaphaneity: somewhat to quite translucent; less transparent with increasing iron

Colors: blue to lavender blue, blue gray, to dark blue; darker with increasing iron

Streak: white to pale blue

Luster: vitreous

Diagnostic Properties: The bluish color, in combination with the hardness and cleavage

characteristic of other amphiboles, is characteristic; the association with blueschist facies

minerals is also helpful


Refractive Indices: nα = 1.594 – 1.626; nβ = 1.612 – 1.642; nγ = 1.618 – 1.648; increase

with iron content


Color/Pleochroism: strongly pleochroic; shades of pale blue, blue, lavender blue, violet,

bluish green, bluish brown, yellow, yellow-green

Birefringence: 0.16 – 0.23; moderate; maximum colors are first order red to second

order green, but the strong colors tend to mask the interference colors

Twinning: simple pairs and lamellar twins occur, but are not common

Optic Sign: Biaxial (–); 2Vx = 10 – 45o; low to moderate 2V; increases with iron content

Other: cross sections show symmetrical extinction; longitudinal sections are length slow

and display relatively small extinction angles (8 – 21o) relative to cleavage traces;

crystals are commonly zoned; may display abnormal interference colors


124o distinguish glaucophane as an amphibole; the combination of blue-violet colors,

length slow optics, relatively small extinction angles and moderate relief are

characteristic; riebeckite is length fast and has deeper colors, higher relief and smaller

extinction angles; tourmaline is uniaxial, so possesses parallel extinction

Occurrences/Associations: Glaucophane occurs primarily in blueschist facies metamorphic

rocks produced by moderate to high pressure, relatively low temperature (high P/T)

metamorphism along Franciscan trajectories in subduction zones in trench-arc systems, where

rocks are forced downward into high pressure depths while remaining relatively cool.

Glaucophanee is commonly associated with lawsonite, pumpellyite, kyanite, muscovite,

pyrophyllite, chlorite, zoisite, aragonite and albite-rich plagioclase in pelitic, mafic, and rare

calcareous rocks formed under blueschist facies conditions (6-12kbar, 300-400oC).

Uses: Glaucophane has no commercial value.

GOETHITE:

Composition: α-FeOOH; an oxyhydroxide mineral; polymorphic with lepidochrosite (β-

FeOOH); see also limonite


90o, γ = 90

o

Crystal Habit: rare prismatic to bladed crystals; more commonly acicular or scaly; commonly in

colloform (reniform-stalactitic) aggregates with fine, radiating crystals; sometimes oolitic; also

massive to granular aggregates and as disseminated crystals


Hardness: 5 – 5½; low side of hard


Cleavage/Fracture: one perfect pinacoidal {010} cleavage; generally not discernible

Diaphaneity: slightly to barely translucent

Colors: yellow-brown to brownish black

Streak: yellow-brown, sienna; may have red tint

Luster: submetallic-adamantine to earthy

Diagnostic Properties: distinguished from hematite by goethite’s yellow-brown streak




Color/Pleochroism: pleochroic with yellow, orange and brownish orange colors

Birefringence: 0.138 – 0.140; extreme, but color obscures high order colors

Optic Sign: Biaxial (–); 2Vx = 0 – 27o; small 2V

Diagnostic Properties: the yellow-orange color, biaxial (–) optics and greater

transparency distinguish goethite from hematite which possesses a deeper red-brown

color, is uniaxial and is nearly opaque; limonite is isotropic, but goethite is often a major

component of those microcrystalline aggregates; goethite closely resembles

lepidochrosite, but the latter has a much larger 2V, if a figure can be obtained; XRD and

other techniques may be required for adequate identifications

Occurrences/Associations: Goethite is a major component of limonite which is commonly

produced by the weathering of iron-bearing minerals and rocks near Earth’s surface and

contributes significantly to the yellow to brown colors common to many soils and to the stains of

that color common on joint surfaces. It is also a common mineral in bog irons, “manganese

nodules”, sedimentary ironstones and banded iron formations. Small amounts occur in low-

temperature hydrothermal veins.

Uses: Goethite (as well as lepidochrosite) is a significant ore of iron (Fe), the primary material

used in the manufacture of a wide range of steel alloys for the manufacture of tools, appliances,

automobiles, trucks, ships, weapons, bridges and building supports. It is also used in the

production of cast iron for cookware, pipes and furnaces and of wrought iron for fences and patio

furniture. Iron is widely used as a dietary supplement, especially for people with symptoms of

anemia.

GOLD:

Composition: Au; native element (native metal); silver (Ag) and copper (Cu) are common

impurities; a complete solid solution series exists between gold and silver; silver-rich gold is

called electrum.

Crystal System: Isometric (4/m/bar32/m): a = 4.71Ǻ, b = 4.71Ǻ, c = 4.71Ǻ; α = 90 o, β = 90

o,

γ = 90o

Crystal Habit: filiform or equant, the latter poorly formed octahedral {111} dodecahedral {011}

and cubic {001} crystals; arborescent or dendritic aggregates; also as disseminated flakes or

rounded detrital “nuggets” in placer deposits


Hardness: 2½-3 (soft)



Diaphaneity: opaque

Colors: yellow-gold; paler with silver (Ag) and reddish with copper (Cu) impurities

Streak: gold-yellow

Luster: metallic

Other: sectile, malleable, ductile

Diagnostic Properties: gold color, gold yellow streak, sectility, malleability and extreme

specific gravity distinguish gold from brass yellow colored sulfides such as pyrite and

chalcopyrite

Microscopic Properties: Opaque; golden yellow in reflected light

Occurrences/Associations: Gold forms principally in low temperature hydrothermal veins

associated with granitoid/rhyolitic-dacitic igneous rocks where it occurs with quartz, pyrite and

other epithermal sulfide minerals. In addition, substantial amounts of gold occur in quartz veins

in low-grade metamorphic terranes and contact metamorphic aureoles. Gold also occurs in

detrital sediments such as placer deposits derived from such source rocks.

Uses: Gold is admired for its beauty and rarity. Gold’s primary use is monetary, in the form of

gold coins and as a hedge against inflation and political uncertainty. Its malleability and ductility

permit it to be used in rings and bracelets and to be hammered into fine gold leaf for plating

ornamental objects. Its metallic properties make it important as an industrial metal in electronic

devices including computers.

GRAPHITE:

Composition: C; polymorphic with diamond which is stable at higher pressures

Crystal System: Hexagonal (6/m2/m2/m): a = 2.46Ǻ, b = 2.46Ǻ, c = 6.71Ǻ; α = 120o, β = 120

o,

γ = 90o

Crystal Habit: platy to scaly crystals; typically in massive to foliated aggregates or as

disseminated grains


Hardness: 1-2; very soft, easily marks paper or flesh

Specific Gravity: 2.2; relatively low

Cleavage/Fracture: one orientation of cleavage {0001}; typically indistinct in

aggregates, visible only when it occurs in flexible sheets

Diaphaneity: opaque

Colors: dark gray to black

Streak: dark gray to black

Luster: typically metallic to submetallic; less commonly dull-earthy

Other: greasy feel as soft material separates onto hand; sectile

Diagnostic Properties: Color, softness and greasy feel are distinctive; molybdenite is

similar but has a greenish-black streak and is a lighter shade of gray.

Microscopic Properties: Opaque; white-gray in reflected light; rather low reflectance

Occurrences/Associations: Graphite is a common mineral in low- to medium-grade

metamorphic rocks, including pelitic rocks such as slate, phyllite and some schist and in

calcareous rocks such as marble and skarn. Most graphite occurs in Phanerozoic rocks where it

is the product of the metamorphism of organic carbon initially deposited in sediments.

Metamorphosed coal beds may also contain graphite. Rare occurrences of graphite are reported

from igneous rocks and hydrothermal veins have been reported. Much graphite is produced

artificially in furnaces that convert coal to graphite.

Uses: Graphite has many uses. Mixed in various proportions with clay, it is a major component

of pencil leads. It is used as a dry lubricant for locks and other devices and as an additive in wet

lubricants such as lubricating oils. Graphite also is used in refractory crucibles for the production

of steel and in electrical resistors in electronic devices such as batteries, electrodes and generator

brushes.

GROSSULAR ( GROSSULARITE); see GARNET

GYPSUM:

Composition: CaSO4•2H2O; a hydrated sulfate mineral


o, γ =

90o

Crystal Habit: the variety of gypsum called Selenite occurs as large, cleavable individual

crystals that are typically tabular to bladed; these are often twinned (simple “swallow-tail”

crystals) and occur in divergent to roseiform aggregates (as in “desert rose”); the variety of

gypsum called Satinspar is composed of parallel acicular to capillary crystals in fibrous

aggregates that commonly form in veins with crystal long axes perpendicular to fracture walls;

the variety of gypsum called Alabaster is composed of small randomly oriented crystals in

compact massive to granular aggregates


Hardness: 2; soft

Specific Gravity: 2.32; low-moderate

Cleavage/Fracture: one perfect pinacoidal cleavage {010} along which selenite cleaves

into thin sheets; a less perfect, very good pinacoidal {100} cleavage at right angles to the

first; two less distinct prismatic {bar111} good cleavages that are not at right angles to

each other nor two the first two cleavages; so single crystals of selenite possess one

perfect cleavage, one less perfect cleavage at right angles to it and two less distinct

cleavages; in alabaster and satinspar the cleavages are generally obscured by the

aggregate habit

Diaphaneity: generally transparent in selenite; somewhat translucent in alabaster and

satinspar

Colors: commonly colorless to white or gray; may be pale pink or yellow

Streak: white

Luster: vitreous in selenite; silky in satinspar; dull to pearly in alabaster

Other: commonly alters, by dehydration, to anhydrite (CaSO4) and forms by the

hydration of anhydrite

Diagnostic Properties: All three varieties are distinctively soft and possess a rather low

specific gravity; the cleavage and swallow-tail twinned monoclinic crystals help to

distinguish selenite; the fibrous habit is characteristic of satinspar and the massive to

granular habit characterizes alabaster; anhydrite is harder and has a higher specific

gravity



Relief: low, negative


Birefringence: 0.010; low with maximum first-order gray and white colors

Twinning: simple, paired twins are common in selenite; multiple and penetration twins

also occur

Optic Sign: Biaxial (+), 2Vz = 58o; on the high side of moderate

Diagnostic Properties: the low, negative relief, low birefringence, biaxial (+) optics with

a moderately high 2V and association with other evaporite minerals are characteristic; in

coarsely crystalline selenite, the cleavage is also a useful diagnostic property as is the

fibrous habit of satinspar; anhydrite is easily distinguished by its higher refractive

indices, positive relief, much higher birefringence and excellent pseudocubic cleavage;

polyhalite possesses positive relief and significantly higher birefringence

Occurrences/Associations: Gypsum is the most abundant mineral in marine evaporite

sequences where it precipitates from sea water or sediment pore water that is undergoing

significant desiccation by net evaporation in a restricted sea or sabkha environment in a warm,

dry climate or by the hydration of anhydrite precipitated under similar conditions. Gypsum is an

important constituent of the cap rocks of salt domes. In both the preceding cases, gypsum is

associated with anhydrite, halite and calcite and in salt domes with native sulfur; smaller

amounts of gypsum form by precipitation from desert lakes undergoing desiccation by net

evaporation, as efflorescence produced by evaporation of capillary water in desert soils and as

veins precipitated from saline groundwater; rare gypsum occurs around volcanic fumaroles

where sulfur vapors react with limestone and in the oxidized portion of hydrothermal deposits

Uses: Ground up, significantly dehydrated gypsum is the major raw material for plaster of Paris,

also called stucco. When mixed with water, fibers and other additives and dried it is extruded

and dried to form sheets of wallboard or sheetrock that set when the mixture dries and gypsum

crystals form. These sheets are used to cover the walls of many houses and other buildings.

Gypsum is also used to retard the setting of cement until it can be worked into the appropriate

forms, as a soil conditioner, and in producing casts (for broken bones). Alabaster is used as a

soft, easy to work, sculpting stone

HALITE:

Composition: NaCl; halide mineral; isostructural with sylvite (KCl), very limited solid solution,

and periclase (MgO), a common mineral only in the mantle


o, γ

= 90o

Crystal Habit: equant cubes {001}; less common octahedral; massive to granular aggregates;

hopper crystals form when edges and corners grow faster than face centers


Hardness: 2½

Specific Gravity: 2.16; low

Cleavage/Fracture: three orientations at 90o; cubic {001}


Colors: colorless to white when pure; may be gray, yellow, orange, red, or blue

Streak: white

Luster: vitreous on fresh surfaces; cloudy on partially dissolved surfaces

Other: salty taste; highly soluble

Diagnostic Properties: The cubic cleavage, transparency and salty taste are highly

indicative of halite; sylvite has a somewhat bitter taste



Relief: very low relief, positive, but so soluble that special techniques required for thin

section

Color/Pleochroism: colorless, but may be colored reddish by inclusions of hematite and

often contains significant amounts of fluid inclusions, making it “cloudy”

Birefringence: none, since isotropic

Optics: isotropic

Diagnostic Properties: again, must be prepared using techniques that do not readily

dissolve it; if used, the perfect cubic cleavage, isotropic nature, extremely low relief and

association with other evaporite minerals serve to distinguish it; sylvite is similar, but has

much lower refractive indices and higher negative relief

Occurrences/Associations: Most halite forms by the evaporation of sea water in restricted seas

in warm, dry climates where it is associated with other evaporate minerals including gypsum,

anhydrite, sylvite and rarer species. Carbonate minerals such as calcite, aragonite and dolomite

and clay minerals are commonly associated with these sedimentary deposits. Halite is the active

component of salt domes formed when, after deep burial, the halite flows upward as a diapir.

Smaller amounts of halite are produced by the evaporation of saline lakes where it is associated

with borates such as kernite and ulexite, sulfates such as gypsum and anhydrite and carbonates

such as calcite and dolomite. Halite is also produced in artificial evaporation ponds in many

coastal areas in regions with warm, arid climates by using flood gates to fill the ponds with sea

water or by pumping salt dissolved from salt domes into the ponds.

Uses: Halite’s most familiar use is as table salt to enhance food flavors. It is also widely used as

a preservative in foods such as cheese, salted fish and meats. Another familiar use is in the

deicing of roads, sidewalks and airplanes. However, its major use is in the production of chlorine

and sodium-based chemicals used in a variety of applications including the processing of metals.

Salt is also used in the curing of leather and in the manufacture of paper and drugs.

HEDENBERGITE:

Composition: CaFeSi2O6; single-chain inosilicate; a clinopyroxene; forms a complete solid

solution series with diopside (CaMgSi2O6); one classification scheme recognizes diopside

(>Di90), salite (Di50–90), ferrosilite (Di10–50) and hedenbergite (< Di10); these minerals also

display extensive solid solution with augite [(Ca,Na)(Mg,Fe,Fe,Al,Ti)Si2O6]; manganese

(Mn+2

) may also substitute in significant amounts for iron (Fe+2

)


o, γ = 90

o


cross sections; may also show prominent parting; commonly in granular aggregates or coarse

cleavable aggregates or as disseminated crystals; less commonly as acicular crystals in radiating

aggregates


Hardness: 5 - 6; hard

Specific Gravity: 3.5 – 3.6; moderate plus


o);


Diaphaneity: moderately translucent to barely transparent

Colors: dark greenish to dark brownish black to black

Streak: gray

Luster: vitreous

Other: may contain exsolution lamellae; commonly alters to chlorite, biotite, hematite,

limonite, carbonates and clay minerals

Diagnostic Properties: hedenbergite cannot be distinguished from other clinopyroxenes

of similar dark color; they are distinguished from amphiboles, such as hornblende, by

their crude, nearly right angle cleavage




Color/Pleochroism: pale green, deep green, blue green, pinkish purple, brownish yellow,

yellow and yellow-green; somewhat pleochroic

Birefringence: 0.018 – 0.034; moderate; maximum first order red to second order red

colors


Optic Sign: Biaxial (+); 2Vz = 55 – 70o; moderate to high 2V


cleavage near right angles distinguish hedenbergite from amphiboles; diopside is similar,

but possesses lower refractive indices and relief and possesses paler colors; augite is

similar in most respects, but augite generally possesses paler colors (except ferroaugite) a

lower 2V and lower refractive indices and a different association; pigeonite has a smaller

2V (< 32o);

Occurrences/Associations: Hedenbergite is an uncommon mineral. It occurs mostly in contact

metamorphic skarns produced by iron-rich hydrothermal fluids. It is also a constituent of some

greenstones. Hedenbergite also occurs, often with fayalite-rich olivine, in iron-rich igneous

rocks where it crystallizes from melts (e.g., tholeiitic magma) that have followed an extreme

iron-enrichment trend during magmatic differentiation. Hedenbergite has also been reported

from chondrules in chondritic meteorites.

Uses: Hedenbergite has no commercial value.

HEMATITE:

Composition: Fe2O3; an oxide mineral; some titanium (Ti+4

), paired with (Fe+2

), typically

substitutes for iron (Fe+3

); polymorphic with maghaemite (maghemite).

Crystal System: Hexagonal (bar32/m): a = 5.04Ǻ, b = 5.04Ǻ, c = 13.77; α = 120o, β = 90

o, γ =

90o

Crystal Habit: individual crystals typically platy-tabular with hexagonal outlines; also bladed;

coarsely crystalline hematite is called specularite; colloform (botryoidal-reniform) aggregates

with radiating acicular-capillary crystals and rarely roseiform with tabular crystals; also abundant

as fine earthy masses and granular and oolitic aggregates


Hardness: 5 - 6; hard; but may appear softer in earthy masses

Specific Gravity: 5.2; high; but may appear less in loosely aggregated masses

Cleavage/Fracture: none, but basal and prismatic partings occur

Diaphaneity: nearly opaque

Colors: steel gray for specularite; red for earthy and oolitic aggregates

Streak: brick red or red brown; specularite may give a black component as scaly crystals

are separated from sample

Luster: metallic for specularite; dull or earthy for finely crystalline aggregates

Other: commonly alters to limonite

Diagnostic Properties: the brick red streak distinguishes hematite from goethite,

limonite, magnetite and ilmenite; specularite is distinguished from ilmenite and magnetite

by its lack of magnetism

Microscopic Properties: very nearly opaque; grayish white with bluish tint in reflected light


Relief: extreme, but nearly opaque

Color/Pleochroism: very nearly opaque; deep red to red-brown tint at thin edges

Birefringence: 0.28; high, but not evidenced due to opacity of hematite

Twinning: lamellar twins occur, again difficult to discern

Optic Sign: Uniaxial (+); good luck getting a figure

Other: commonly observed as an alteration (oxidation) product of other iron-bearing

minerals

Diagnostic Properties: the nearly opaque, yet anisotropic character and deep red to red

brown color are distinctive; limonite is isotropic and typically possesses a more yellow-

brown color, though hematite can be a significant constituent; goethite tends to be

yellow-orange, a bit more transparent and is biaxial; in finely-crystalline aggregates,

mixtures occur and XRD is required to sort things out

Occurrences/Associations: Hematite occurs abundantly in a variety of geologic settings. It is a

common mineral in largely sedimentary to metasedimentary Precambrian banded iron formations

(BIF) and their metamorphosed equivalents, where it is associated with chert, magnetite,

greenalite, minnesotaite siderite, dolomite, pyrite and clay minerals. It is also common in largely

Phanerozoic sedimentary ironstones where it is associated with limonite, magnetite, siderite,

chamosite, and calcite. It occurs in some contact metamorphic deposits (with magnetite) and in

iron-rich schists. Hematite is also commonly produced by the weathering, diagenetic alteration

and hydrothermal alteration of iron-bearing minerals in almost any rock type. The red color of

many sedimentary rocks and soils is caused by the presence of hematite. Hematite is significant

cement in some sedimentary rocks. Hematite is also an accessory mineral in some felsic igneous

rocks and forms in some hydrothermal deposits.

Uses: Hematite is a principal ore of iron used in the manufacture of a wide range of steel alloys

for the manufacture of tools, appliances, automobiles, trucks, ships, weapons, bridges and

building supports. It is also used in the production of cast iron for cookware, pipes and furnaces

and of wrought iron for fences and patio furniture. Iron is widely used as a dietary supplement,

especially for people with symptoms of anemia.

HEULANDITE:

Composition: CaAl2Si7O18•6H2O; a tectosilicate mineral; a zeolite mineral; up to 20%

substitution of sodium (Na+1

) for calcium (Ca+2

) is common, so could be written as: (Na0.2Ca0.8)

and is balanced by substitution of silicon (Si+4


)

Crystal System: Monoclinic (2/m): a = 7.96Å, b = 17.95Å, c = 7.44; α = 90o, β = 92

o, γ = 90

o;

crystals are pseudo-orthorhombic (given β near 90o)

Crystal Habit: occurs as tabular to bladed pseudo-orthorhombic crystals; often as drusy to

granular aggregates of such crystals or as more finely-crystalline massive aggregates or





Cleavage/Fracture: one orientation of pinacoidal {010} cleavage; perfect


Colors: typically colorless to white or gray; also shades of yellow, red, pink, brown

Streak: white

Luster: vitreous to pearly on the cleavage surfaces

Diagnostic Properties: the tabular, pseudo-orthorhombic crystals, hardness and single

set of perfect cleavage with pearly luster are characteristic; finely crystalline aggregates

require XRD analysis for proper identification


Refractive Indices:



Birefringence: 0.01 -0.11; very low to low; maximum colors first order grays to yellow

Optic Sign: Biaxial (+); 2Vz = 0 – 70o; but typically 30

o ± 10

o; moderate

Other: sections that display the perfect cleavage are characterized by parallel extinction;

others possess angular extinction

Diagnostic Properties: the negative relief and low birefringence are typical of most

zeolite minerals; stilbite is similar, but is biaxial (–) and typically possesses a sheaf-like

habit, rather than tabular-bladed crystals; chabazite is pseudo-rhombohedral, with 3 sets

of pseudocubic cleavage; natrolite has slightly higher birefringence and is orthorhombic,

typically with a higher 2V; laumontite is biaxial (–) with three sets of cleavage; analcime

is isotropic

Occurrences/Associations: Heulandite is a relatively widespread zeolite mineral. Coarse

crystals are especially common in cavities, including vesicles, in mafic/basic rocks such as basalt

and, less commonly, intermediate volcanic rocks such as andesite. Finely crystalline heulandite

occurs less commonly in altered felsic volcanic rocks. Heulandite is a common mineral in very

low-grade regional metamorphic rocks of the zeolite facies and in some contact metamorphic

aureoles. Heulandite is also reported from alkaline soils formed in desert environments and from

low-temperature hydrothermal deposits and hot springs.

Uses: Heulandite, like most natural and synthetic zeolites, is used, after heating and dehydration,

to produce materials that remove water vapor from carbon dioxide, refrigerants such as Freon

and from many organic chemicals used in industrial applications. It is also used as a molecular

sieve used to separate molecules of different sizes, as in separating nitrogen from air to produce

nearly pure oxygen gas. Because zeolites can selectively absorb selected molecules, they are

widely used in applications that include reducing the hardness of water for cleaner laundry by the

removal of calcium, removing heavy metals from mine waters and industrial waste and

radioactive isotopes from nuclear waste, in the treatment of wastewater by the removal of

undesirable dissolved species such as ammonia and in the cleaning up of oil spills.

HORNBLENDE:

Composition: (Na,K)0-1Ca2(Mg,Fe,FeAl)5(Al,Si)2Si6O22(OH,F)2; a double-chain inosilicate; an

amphibole group mineral; may be thought of as a complex solid solution of several end member

amphiboles with various amounts of the principal cations; oxyhornblende is a form of hornblende in

which the ferrous iron (Fe+2

) has been oxidized to ferric iron (Fe+3

) and hydroxyl ion (OH-1

) replaced by

oxygen (O-2

), typically in volcanic rocks


o, γ = 90

o

Crystal Habit: slender, bladed prismatic; less commonly stubby prismatic; 6-sided, diamond-

shaped cross sections with beveled edges; sometimes acicular in fibrous aggregates; more

typically as massive to granular aggregates and disseminated crystals


Hardness: 5-6

Specific Gravity: 3.0 – 3.6; moderate to moderate plus

Cleavage/Fracture two orientations of prismatic cleavage {110} not at right angles (56o

and 124o); good to very good; splintery appearance is common


Colors: typically dark green to black; also green or brown

Streak: gray to greenish gray

Luster: vitreous

Other: commonly altered to biotite or chlorite

Diagnostic Properties: the hardness and cleavage are distinctive of amphibole; the dark

color distinguishes hornblende from most other amphiboles; ferro-actinolite and

hornblende can be difficult to distinguish


Refractive Indices: nα = 1.60-1.70; nβ = 1.61-1.72; nγ = 1.62-1.73; increasing with

increasing iron content

Relief: moderate to moderate-high; positive

Color/Pleochroism: shades of green and brown with the former predominate in green

hornblende and the latter predominant in brown hornblende; pleochroism increases with

iron content; green hornblende is pale green, gray green, yellow green, yellow, deep

green, green, olive green, deep bluish green, blue green; brown hornblende is greenish

yellow, yellow brown, yellow, greenish brown, pale brown, sepia brown, reddish brown

Birefringence: (0.014-0.034); low-moderate to moderate; upper first order to upper

second order colors

Twinning: simple paired and lamellar twins are common;

Optic Sign: Biaxial (–); 2Vx = 55 – 90o; high to very high 2V; rarely Biaxial (+) with

very high 2V; 2V increases generally with iron content

Other: cross-sections showing the two cleavages display symmetrical extinction; may be

zoned; pale green hornblende (uralite) commonly forms reaction rims around pyroxene


124o distinguish hornblende as an amphibole; the green-brown pleochroism, (–) optic

sign, high 2V and moderate birefringence are characteristic of hornblende; actinolite

possesses similar, if paler, less pleochroic, colors, higher birefringence, lower relief and

smaller extinction angles and occurs almost exclusively in metamorphic rocks;

oxyhornblende has deeper, more strongly pleochroic colors, higher birefringence and

smaller extinction angles; biotite possesses a single cleavage, higher birefringence and

near parallel, birdseye extinction

Occurrences/Associations: Hornblende is the most common and widely distributed

amphibole group mineral. It is an essential constituent of a wide variety of felsic, acidic

and intermediate rocks including granitoids, syenite and diorite and equivalent volcanic

rocks, where it (and oxyhornblende) is a common phenocryst phase. It is less abundant,

but still significant constituent of mafic/basic and even ultramafic/ultrabasic such as

hornblende gabbro and hornblendite. Hornblende is also an important constituent of

medium-grade, mafic and quartzofeldspathic rocks such as amphibolite, hornblende

schist and gneiss formed in the epidote amphibolite, amphibolite and hornblende hornfels

facies. It also occurs, less commonly, in some calcareous and pelitic rocks.

Uses: Hornblende has little economic value. Rocks that contain hornblende are used as

dimension stone for countertops and facings.

HYPERSTHENE: see ORTHOPYROXENE

ICE:

Composition: H2O; an oxide mineral (X2O group)


90o, γ = 120

o

Crystal Habit: occurs as lacey, skeletal stellated to dendritic hexagonal crystals in snowflakes

and frost; more abundantly as granular to massive crystal aggregates in glaciers or on the

surfaces of frozen bodies of water


Hardness: 1½; soft

Specific Gravity: 0.92; extremely low (buoyant in water!)

Cleavage/Fracture: no cleavage; excellent conchoidal fracture

Diaphaneity: transparent to very translucent

Colors: colorless to white, when pure; bluish tint in thick accumulations

Streak: white

Luster: vitreous

Other: unusually low melting temperature makes it unstable in many surface

environments where it melts at 0oC and sublimates at lower temperatures; therefore

always cold to the touch

Diagnostic Properties: low hardness, extremely low specific gravity, low melting

temperature and cold feel, white color, vitreous luster and conchoidal fracture render ice

difficult to confuse with any other mineral

Microscopic Properties: thin-sections cannot be prepared by normal methods because of low

melting point

Refractive Indices: nω = 1.309; nε = 1.311;

Relief: very high, negative


Birefringence: 0.002; extremely low; nearly isotropic

Twinning: basal deformation twins are common


Diagnostic Properties: melts at room temperature

Occurrences/Associations: Because of its low melting temperature, ice is a sedimentary mineral

that forms at low temperatures in surface and near-surface environments. It typically forms by

precipitation from water vapor in the atmosphere in the form of snow, hail or frozen rain. Thick

snow accumulations commonly recrystallize (a diagenetic, not metamorphic process) to form

glacial ice deposits whose thickness may exceed 5km and blanket extensive areas, especially

during “ice ages”. Ice forms as frost when water vapor freezes on surfaces. Ice also forms when

liquid water freezes on the surface of lakes, ponds, rivers and the ocean. Lastly ice forms when

liquid soil water freezes, both in areas of seasonal freezing and in areas of permafrost, as well as

when liquid freezes in rock fractures, a major agent in the disintegration of rock bodies by frost

action.

Uses: The major use of ice is to cool alcoholic and non- alcoholic beverages. Prior to the

invention of refrigeration, ice cut from ponds was used to preserved food by keeping it cool in an

ice box; the use of ice in travel coolers is similar. Ice is also used to construct structures, e.g.

igloos, in cold regions, to replenish snow on skiing courses and to reduce swelling after injuries.

Ice is also used in sculptures, as a surface in ice-skating and hockey rinks and to lower patients’

body temperature during medical procedures.

IDOCRASE: old name for VESUVIANITE

ILLITE:

Composition: K0.8Al2(Al0.8Si3.2)(OH)2; a phyllosilicate mineral; a 10Å clay mineral with a T-O-

T structure, similar to that in muscovite; similar clay minerals that contain more calcium,

sodium, iron and magnesium include glauconite and celadonite and small amounts of these may

be in solid solution with illite

Crystal System: Monoclinic (2/m): a = 5.18Å, b = 9.02Å, c = 9.98Å; α = 90o, β = 90+

o, γ = 90

o

Crystal Habit: typically as very small (< 4μm), platy microcrystals; typically in finely

crystalline, massive aggregates or as disseminated crystals or grains; common component of

mixed layer clays


Hardness: 1 - 2; soft to very soft


Cleavage/Fracture: one orientation of basal pinacoidal {001} cleavage; perfect; almost

always indistinct

Diaphaneity: nearly transparent in crystals to barely translucent in massive aggregates

Colors: colorless to pale gray or green, but may be colored by impurities such as limonite

and hematite may render it tan, yellow-brown or reddish

Streak: white to pale gray

Luster: dull, earthy

Other: possesses a somewhat “sticky”, cohesive feel when pure

Diagnostic Properties: Illite is distinguished from other clay minerals with which it is

commonly mixed only by advanced methods such as XRD





Birefringence: 0.028 – 0.032; maximum high second order colors; typically somewhat

lower in microscopic flakes

Optic Sign: Biaxial (–): 2Vx = 0 - 10o; low 2V; figures rarely obtained

Diagnostic Properties: the microscopic, platy crystals, lack of color, low-moderate relief

and moderate birefringence distinguish illite; kaolinite possesses lower birefringence and

a larger 2V (if a figure can be obtained); smectite clays are difficult to distinguish from

illite, unless they are iron-rich, in which case they posses yellow-brown-green

pleochroism; there is a large variety of clay minerals and most cannot be distinguished

optically and require alternate techniques for accurate identification

Occurrences/Associations: Illite forms chiefly from the alteration of aluminum silicate minerals

especially potassium feldspar, but also plagioclase, feldspathoids and a host of metamorphic

minerals. It is produced either by hydrothermal alteration of such minerals or by weathering. It

is therefore an abundant constituent of soils and of detrital sedimentary rocks, especially shale

and mudstone.

Uses: Illite, as an important constituent of clay, is used in the production of bricks, ceramics,

paper, plastics, cosmetics and refractory materials. Because of its very low permeability,

compacted clay is commonly used to line hazardous waste sites, containment ponds and city

dumps.

ILMENITE:

Composition: FeTiO3; an oxide mineral; ferric iron (Fe+3

) in simultaneous substitution for

titanium (Ti+4

) and ferrous iron (Fe+2

); in this way forms a coupled solid solution series with

hematite

Crystal System: Hexagonal: trigonal (bar3): a1 = 5.09Ǻ, a2 = 5.09Ǻ, c = 14.09Ǻ; α = 120o, β =

90o, γ = 120

o

Crystal Habit: tabular to platy crystals with rhombohedral truncations; commonly in massive to

granular aggregates or as disseminated grains and crystals


Hardness: 5½ - 6


Cleavage/Fracture: none; but parting due to twinning may be discernable

Diaphaneity: opaque

Colors: black

Streak: black

Luster: metallic to submetallic

Other: commonly intergrown with magnetite; alters to limonite and leucoxene (a

mixture of iron and titanium oxides)

Diagnostic Properties: Ilmenite is characterized by its black color, black streak and

hardness; it is distinguished from specular hematite by its black streak and from

magnetite by its weaker magnetism (if any)

Microscopic Properties: opaque; brownish with a pale pink or violet tint in reflected light

Occurrences/Associations: Ilmenite occurs widely as a disseminated accessory mineral in both

igneous and metamorphic rocks. It also in concentrated in the heavy mineral populations of

detrital sediments derived from such source rocks. Large concentrations of ilmenite occur as

segregation products in mafic/basic and ultramafic/ultrabasic rocks such as gabboro, norite and

peridotite and associated rocks such as anorthosite where it commonly occurs with magnetite. It

occurs less commonly in hydrothermal veins.

Uses: Ilmenite is a major ore of titanium (Ti) which is used with iron to strengthen steel and in

alloys with aluminum, molybdenum and vanadium that are used in aircraft engines, missiles and

spacecraft. Because of its resistance to corrosion, it is used in propeller shafts and riggings on

boats. Because it is physiologically inert and can integrate with bones, titanium is a choice

material for joint replacement and tooth implants. Because of its light weight and strength,

titanium is used in alloys for golf clubs and eyeglass frames. Titanium is used most extensively

as a pigmenting agent in white paints and plastics.

JADEITE:

Composition: NaAlSi2O6; single chain inosilicate; clinopyroxene; some coupled substitution

(Ca+2

for Na+1

) and (Mg+2

and/or Fe+2

for Al+3

) commonly occurs

Crystal System: Monoclinic (2/m): a = 9.5, b = 8.6, c = 5.2; α = 90o, β = l08

o, γ = 90

o

Crystal Habit: crystals are stubby prismatic to prismatic with squarish four to eight-sided cross-

sections; also acicular; typically in massive or granular aggregates; also fibrous




Cleavage/Fracture: two prismatic cleavages {210}, near 90o (87

o and 93

o); good only;

massive aggregates do not display cleavage and are very tough

Diaphaneity: nearly transparent to somewhat translucent

Colors: typically various shades light apple green to emerald green; also colorless, white

or bluish green; mottled color variations in one sample common

Streak: white to pale green-gray


Diagnostic Properties: green, sometimes mottled, color and toughness distinguish the

massive aggregates from other minerals;




Color/Pleochroism: colorless to very pale green; deeper shades of green and yellow in

iron-rich varieties

Birefringence: 0.08 – 0.21; low-moderate; maximum colors are first order reds to second

order green

Twinning: simple paired twins and lamellar twins occur

Optic Sign: Biaxial (+) or, less commonly, (–); 2Vz = 60 - 90o; 2Vx = 84 - 90

o; high 2V

Other: cross-sections (showing near right angle cleavage) display symmetrical

extinction; longitudinal sections show one cleavage with maximum extinction angle of

30-41o to cleavage


cleavage near right angles and squarish 4- 8-sided cross sections; acicular-fibrous

varieties do not display the cleavages well, but can be distinguished from nephritic

tremolite-actinolite by jadeite’s higher refractive indices and larger extinction angles;

jadeite is quite similar to both diopside and augite in color, but diopside and augite are

always biaxial (+), possess generally higher birefringence and higher refractive indices

and relief and typically have a smaller 2V; omphacite is closely similar to jadeite, but

occurs in the eclogite facies rather than in the blueschist facies so that the associations are

somewhat different

Occurrences/Associations: Jadeite occurs principally in metamorphic rocks of the blueschist

facies produced by high P/T metamorphism along Franciscan and/or Sanbagawan trajectories in

subduction zones. It is typically associated with minerals such as glaucophane, albite-rich

plagioclase, lawsonite, garnet, aragonite, calcite, quartz, muscovite, zoisite and kyanite

Uses: Jadeite is the principal mineral in jade (some jade is composed of the amphibole called

nephrite) that is widely used in jewelry and carved decorative objects and utensils.

JASPER: see CHERT

KAOLINITE:

Composition: Al2Si2O5(OH)4; a phyllosilicate mineral; a 7Å clay mineral with a T-O structure;

similar clay minerals with a slightly different layer stacking arrangement are dickite, nacrite and

halloysite

Crystal System: Triclinic (ī): a = 5.14Å, b = 8.94Å, c = 7.38Å; α = 91.5o, β = 104.7

o, γ = 90

o


crystalline, massive aggregates


Hardness: 2; soft


Cleavage/Fracture: one orientation of basal pinacoidal {001} cleavage; perfect; almost

always indistinct

Diaphaneity: somewhat to barely translucent in massive aggregates

Colors: white, but may be colored by impurities such as limonite and hematite may

render it tan, yellow-brown or reddish

Streak: white

Luster: earthy to chalky; sometimes pearly

Other: typically possesses a “sticky”, cohesive feel

Diagnostic Properties: The white color, low hardness, pearly-earthy luster and “sticky”

feel are characteristic, but distinguished from other clay minerals with which it is

commonly mixed only by advanced methods such as XRD





Birefringence: 0.005 – 0.007; quite low; first order gray

Optic Sign: Biaxial (–): 2Vx = 24 – 50o; low to moderate 2V; figures rarely obtained

Diagnostic Properties: the microscopic, platy crystals, lack of color, low-moderate relief

and low birefringence distinguish kaolinite; illite and smectite possess significantly

higher birefringence and a smaller 2V (if a figure can be obtained); because of their

microcrystalline nature, clays are difficult to distinguish from one another; there is a large

variety of clay minerals and most cannot be distinguished optically and require alternate

techniques for accurate identification

Occurrences/Associations: Kaolinite forms chiefly from the alteration of aluminum silicate

minerals especially feldspars, but also feldspathoids and a host of metamorphic aluminum

silicate minerals. It is produced either by hydrothermal alteration of such minerals or by

weathering, especially in warm, humid climates with acidic soil waters. It is therefore a

common constituent of soils and of detrital sedimentary rocks, especially shale and mudstone.

Uses: Kaolinite is an economically important clay mineral. It is an important ingredient in many

cosmetic products including It is used in the manufacture of quality china and porcelain ware (as

china clay or kaolin), in the production of ceramic products including roofing tiles, bricks and

sewer pipes. It is also used in the production of cosmetics, pharmaceuticals, cement, drilling

mud, paper, paint, rubber and refractory materials to line furnaces.

KERNITE:

Composition: Na2B4O6(OH)2•3H2O; a borate mineral


o, γ =

90o

Crystal Habit: sometimes occurs in nearly equant crystals; more commonly occurs as granular

to coarsely crystalline, cleavable masses


Hardness: 2½ - 3

Specific Gravity: 1.91; quite low

Cleavage/Fracture: two orientations not at right angles (71o and109

o); both pinacoidal

{100} and {001} and perfect; the intersection of these sets can mimic a fibrous habit


Colors: colorless to white or pale gray

Streak: white


Other: may acquire a white, crumbly coating (the mineral tincalconite) by dehydration

after prolonged contact with the atmosphere; slowly soluble in water

Diagnostic Characteristics: distinguished by its two orientations of cleavage not at 90o

that yield long, splintery fragments that mimic a fibrous habit and hardness



Relief: moderate negative


Birefringence: 0.032; high; 3rd

and 4th

order colors

Optic Sign: Biaxial (–); 2Vx = 80o; very high

Diagnostic Properties: a scarce mineral, kernite can be recognized by the combination

of two perfect cleavages not at right angles, moderate negative relief, high birefringence,

biaxial (–) optics with a high 2V, and its association with other continental evaporite

minerals

Occurrences/Associations: Most kernite forms by the alteration of borax produced by

evaporation in enclosed, saline lake in warm, arid climates and as an efflorescent formed by soil

water evaporation under similar conditions. Commonly associated minerals include borax,

ulexite, colemanite, gypsum, anhydrite, halite and a host of rare borate minerals.

Uses: The most important use of kernite is in the production of borax used in manufacture of

glass fibers for use in insulation and textiles. Borax is also used in detergent soaps and as a flux

in metallurgical processes such as welding and smelting. One isotope of elemental boron,

obtained from kernite, is used in the shields of atomic reactors and elemental boron is also used

in fuels for rockets and automobiles and in the high strength plastics used in aircraft

manufacturing.

KYANITE:

Composition: AlAlOSiO4; a nesosilicate (orthosilicate) mineral, polymorphic with andalusite

and sillimanite; small amounts of iron (Fe+3

) may substitute for aluminum (Al+3

)

Crystal System: Triclinic (ī): a = 7.12Å, b = 7.85Å, c = 5.57Å; α = 90o, β = 101

o, γ = 106

o

Crystal Habit: typically in elongate, bladed (to prismatic) crystals which sometimes possess a

“twisted” appearance; mostly as disseminated crystals; less commonly as tabular crystals; occurs

as foliated, reticulated or granular aggregates or as disseminated crystals.


Hardness: 7 on the cleavage; very hard; ± 5 parallel to prism elongation; moderate plus


Cleavage/Fracture: one orientation of pinacoidal {100} cleavage; perfect; a second

pinacoidal {010} cleavage not at 90o (79

o) is good only, so may or may not be

discernible; splintery fracture sometimes observed


Colors: most commonly bluish to bluish gray or bluish white; also gray; rarely greenish;

colors are often somewhat “patchy” or mottled

Streak: white

Luster: vitreous

Other: alters to sericite and chlorite

Diagnostic Properties: The bluish colors, bladed crystals, patchy colors, good cleavage

and significantly different hardness in different directions are characteristic of most

kyanite




Color/Pleochroism: colorless to pale blue; not pleochroic

Birefringence: 0.012 – 0.016 low; maximum first yellow to red colors typical

Twinning: simple paired twins common; lamellar twins occur as well

Optic Sign: Biaxial (–), 2Vx = 78-84o; very high 2V

Diagnostic Properties: the combination of bladed crystals, with excellent cleavage,

relatively high relief, relatively low birefringence, colorless to pale blue character,

biaxial (–) optics with very high 2V and the large extinction angles are characteristic of

kyanite; sillimanite is easily distinguished because it is commonly acicular, possesses

lower relief and slightly higher birefringence, is biaxial (+) with small 2V, is typically

colorless to gray and possesses parallel extinction

Occurrences/Associations: Kyanite is the high pressure polymorph of aluminum silicate and

typically forms in high P/T pelitic metamorphic rocks such as schist, gneiss and granulite. It is

therefore abundant in regional metamorphic rocks produced in the blueschist facies where it is

associated with lawsonite, muscovite, pyrophyllite, pumpellyite, chloritoid, corundum,

glaucophane and garnet. It also occurs in the higher pressure portions of the amphibolite facies

where it is produced along Sanbagawan and Barrovian trajectories and is associated with garnet,

cordierite, staurolite, biotite, muscovite and sillimanite. Kyanite occurs in some eclogites, with

omphacite and pyrope-rich garnet and is found in some ultra-high pressure (UHP) facies rocks.

Kyanite also occurs in peraluminous igneous rocks including granitoids and pegmatite, although

whether or not it is a primary mineral in such rocks is controversial.

Uses: Kyanite is mined extensively to be refined for use as a refractory material used in the

manufacture of the porcelain used spark plugs and high alumina bricks for lining blast furnaces

and kilns. It may also be used as an abrasive.

LABRADORITE: see PLAGIOCLASE

LAZURITE: see SODALITE

LAWSONITE:

Composition: CaAl2(Si2O7)(OH)2•H2O; a disilicate (sorosilicate) mineral; substitutions for

Ca+2

(Na+1

) and Al+3

(Fe+3

, Fe+2

, Mg+2

and Ti+4

) occur on a small scale


90o, γ = 90

o

Crystal Habit: tabular crystals with rectangular cross-sections; also as massive to granular

aggregates and commonly as disseminated crystals




Cleavage/Fracture: two sets of pinacoidal {100} and {010} cleavage at 90o; very good


Colors: commonly bluish gray; also colorless, white, or bluish green

Streak: white

Luster: vitreous to greasy

Diagnostic Properties: The elevated hardness, tabular crystals with rectangular cross-

sections and two perpendicular orientations of cleavage and color are diagnostic as is the

association with other high P/T minerals


Refractive Indices: nα = 1.665; nβ = 1.672 – 1.676; nγ = 1.684 – 1.686

Relief: moderate-high

Color/Pleochroism: colorless to very pale bluish green; may be very weakly pleochroic

Birefringence: 0.020; moderate; maximum colors are first order red to purple

Twinning: simple or lamellar twins common; may be curved

Optic Sign: Biaxial (+); 2Vz = 76 – 87o; high 2V

Other: cross sections that symmetrical extinction; the longitudinal sections are length

slow and display parallel extinction

Diagnostic Properties: the combination of rectangular cross sections, moderate to high

relief; low-moderate birefringence, two sets of cleavage and biaxial (+) optics with high

2V are characteristic, as is the association with high pressure blueschist facies minerals;

andalusite is similar in many properties, but possesses lower birefringence and square

cross sections and occurs in low pressure associations; clinozoite has only one cleavage,

higher refractive indices, inclined extinction, often lower 2V and anomalous interference

colors and zoisite possesses only one cleavage, a lower 2V and often anomalous

interference colors

Occurrences/Associations: Lawsonite most commonly occurs in blueschist facies metamorphic

rocks, formed by moderate to high pressure and low temperature (high P/T) conditions

associated with rocks being rapidly forced downward along Franciscan trajectories to relatively

high pressures, while remaining relatively cool, during subduction in trench-arc systems.

Lawsonite is commonly associated with glaucophane, pumpellyite, kyanite, muscovite,

pyrophyllite, chlorite, zoisite, aragonite and albite-rich plagioclase in pelitic, mafic, and rare

calcareous rocks formed under blueschist facies conditions (6-12kbar, 300-400oC). Less

common lawsonite occurrences have been reported from high pressure, higher temperature

metamorphic rocks of the eclogite facies.

Uses: Lawsonite has no economic value.

LEPIDOCHROSITE: see GOETITE, LIMONITE

LEPIDOLITE: see MUSCOVITE

LEUCITE:

Composition: KAlSi2O6; a tectosilicate mineral; a feldspathoid mineral; small amounts of

substitution of sodium (Na+1

) for (K+1

) and ferric iron (Fe+3


) may occur

Crystal System: Tetragonal (4/m); a1 = 13.05Å, a2 = 13.05Å, c = 13.75Å; α = 90o, β = 90

o, γ =

90o; pseudoisometic crystals common because leucite crystallizes at high temperatures as an

isometric mineral that inverts to tetragonal on cooling

Crystal Habit: Equant; trapezohedral crystals {hhl} resemble “soccer” balls; also as




Specific Gravity: 2.5; moderate (low side of moderate)

Cleavage/Fracture: exists, but poor and so not generally discernable

Diaphaneity: somewhat to quite translucent

Colors: white to gray

Streak: white

Luster: vitreous to dull

Other: commonly occurs as phenocrysts set in aphanitic groundmass of volcanic and

hypabyssal rocks; also disseminated in the groundmass; commonly alters to analcime or

to psuedoleucite which is a mixture of nepheline and potassium feldspar

Diagnostic Properties: the color, hardness, trapezohedral crystals and occurrence as

phenocrysts in volcanic/hypabyssal rocks are distinctive; microscopic crystals in the

groundmass cannot of course be identified by macroscopic techniques


Refractive Index: nω = 1.508 – 1.511; nε = 1.509 -1.511



Birefringence: 0.000 – 0.001; appears isotropic or nearly so

Twinning: complex, concentric lamellar twins, intersect at ~60o

Optic Sign: none, essentially isotropic

Diagnostic Properties: the 8-sided cross sections (through trapezohedra), low negative

relief, lack of color and extremely low (almost isotropic) birefringence are characteristic;

the complex twins are also useful

Occurrences/Associations: Leucite is a scarce mineral, occurring chiefly in alkali-rich

(especially K2O-rich), silica undersaturated rocks formed at or near the surface, including

phonolite and foid syenite. It is typically associated with nepheline, albite-rich plagioclase,

sanidine, aegerine or aegerine-augite, and riebeckite or arvedsonite.

Uses: Leucite has no economic value or use.

LIZARDITE: see SERPENTINE

LIMONITE: see goethite; technically a rock or soil, but commonly discussed with minerals

Composition: Variable; sometimes as FeOOH•nH2O; Limonite is a fine-grained mixture of

different hydroxide, oxyhydroxide and oxide minerals including goethite (α-FeOOH),

lepidochrosite (β-FeOOH), jarosite [KFe3(OH)6(SO4)2], hematite (Fe2O3) and clay minerals;

analogous to bauxite for aluminum hydroxides and oxyhydroxides

Crystal System: not applicable

Crystal Habit: limonite typically occurs in fine-grained, earthy masses and as surficial

encrustations; also as pseudomorphs after other iron-bearing minerals


Hardness: 1 – 5; variable apparent hardness

Specific Gravity: 2.0 – 4.3; low to moderately high apparent specific gravity; depending

on mineralogy and compactness (porosity)

Cleavage/Fracture: none discernible

Diaphaneity: slightly transparent in hand specimen

Colors: variable, but typically “rusty” yellow brown to darker sienna brown

Streak: yellowish brown to orange brown

Luster: dull in earthy masses; small metallic highlights where coarser

Diagnostic Properties: combination of yellow brown to orange brown streak, relative

softness, rusty yellow brown color and earthy luster are distinctive

Microscopic Properties: see goethite; shades of yellow brown to nearly opaque in thin section,

with high indices of refraction characterize limonite mixtures

Occurrences/Associations: Limonite is most commonly produced by the weathering of iron-

bearing minerals and rocks near Earth’s surface. It is also produced by the low temperature

hydrothermal alteration of such minerals. It contributes significantly to the yellow to brown

colors common to many soils and to the stains of that color common on joint surfaces.

Uses: Limonite is an ore of iron (Fe), the primary material used in the manufacture of a wide

range of steel alloys for the manufacture of tools, appliances, automobiles, trucks, ships,

weapons, bridges and building supports. It is also used in the production of cast iron for pipes

and furnaces. Limonite is also used as a pigment. Limonite in the form of “gossan” produced by

the weathering of pyrite and other iron-bearing sulfides was once used by prospectors to locate

underlying hydrothermal vein deposits.

MAGNESITE:

Composition: MgCO3; isostructural with calcite, siderite, rhodochrosite, and smithsonite;

partial solid solution exists with calcite (see high-Mg calcite) and more with rhodochrosite and

smithsonite, all members of the rhombohedral (calcite) group of carbonate minerals; a complete

solid solution exists with siderite; some siderite (Fe) component is common in magnesite as a

result of the abundance of iron in natural environments


= 120o, β = 90

o, γ = 120

o

Crystal Habit: rare equant rhombohedral {10bar10} crystals; typically in compact, massive

microcrystalline aggregates or coarser granular to massive aggregates


Hardness: 3½ - 4½; harder if intergrown with microcrystalline silica or opal

Specific Gravity: 3.0; average; higher where Fe substitutes for Mg

Cleavage/Fracture: 3 orientations at 90o; rhombohedral {10bar10}; compact, earthy

masses display subconchoidal fracture

Diaphaneity: translucent to somewhat translucent in earthy masses

Colors: variable; but white and gray common; also yellow-brown when iron present

Streak: white

Luster: vitreous; to dull in earthy masses

Diagnostic Properties: massive microcrystalline aggregates resemble chert, but are

much softer; cleavable masses resemble dolomite, but have a higher specific gravity



Relief: variable; from low negative to high positive; sections in which both nω and nε are

visible produce a “twinkling” effect as the relief changes when stage is rotated under

plane light

Color/Pleochroism: colorless; iron-rich varieties transitional to siderite may display pale

yellow to brown colors due to the presence of oxidized iton

Birefringence: 0.188; very high to extreme; so high order, faded colors in most

orientations

Twinning: lacks mechanical twinning


Other: extinction is symmetrical with respect to intersecting cleavages



uniaxial (–) optics are characteristic of rhombohedral carbonates such as magnesite;

magnesite is distinguished from dolomite and calcite only with difficulty; magnesite

lacks mechanical twins common to dolomite and calcite and occurs much less commonly

as euhedral rhombohedra than does dolomite; iron-poor magnesite lacks they yellow-

brown color of siderite, but iron-rich varieties are difficult to distinguish from the latter;

siderite possesses higher refractive indices and positive relief in all orientations

Occurrences/Associations: Most magnesite is produced by hydrothermal alteration

(metamorphism) of ultramafic rocks such as peridotites; for example by hydrothermal solutions

derived from heated sea water circulating through ultramafic rocks near the oceanic ridge

system. These deposits typically occur as microcrystalline, earthy masses. Scarcer magnesite

deposits are associated with sedimentary dolostone and evaporite deposits where they are

thought to form by diagenetic or hydrothermal alteration of preexisting carbonate minerals such

as dolomite. Magnesite also occurs as a rare mineral in hydrothermal veins.

Uses: By heating magnesite and driving off carbon dioxide (CO2) magnesium oxide (MgO) is

produced which is used in the production of refractory bricks or linings for furnace and industrial

oven linings. Most magnesium for other uses is currently derived from seawater or groundwater

brines.

MAGNETITE:

Composition: FeFe2O4; an oxide; a spinel group (XY2O4 group) mineral; commonly contains

some titanium (Ti+4

) that substitutes for Fe+3

in the X site with the charge difference balanced by

the simultaneous substitution of Fe+2

for Fe+3

in the Y site, as part of a complete solid solution

series with ulvospinel FeFeTiO4; magnesium (Mg+2

) and manganese (Mn+2

) may substitute for

iron (Fe+2

) and some chromium (Cr+3

) for iron (Fe+3

)


o,

γ = 90o

Crystal Habit: commonly occurs as small, equant octahedral {111} crystals; rarely

dodecahedral {110}; more commonly as massive to granular crystal aggregates or disseminated

crystals


Hardness: 6; hard

Specific Gravity: 5.2; high; less if substantial magnesium is present

Cleavage/Fracture: none; sometimes displays octahedral parting

Diaphaneity: opaque

Colors: black

Streak: black

Luster: metallic

Other: strongly ferromagnetic; the variety known as “lodestone” acts as a natural magnet

Diagnostic Properties: the strong magnetism is most diagnostic; the black color, black

streak, metallic luster, lack of cleavage and high hardness are characteristic; ilmenite is

similar, but much more weakly magnetic; chromite has a brownish streak and pitchy

luster

Microscopic Properties: opaque; brownish gray in reflected light

Occurrences/Associations: Magnetite is widely distributed in igneous, sedimentary and

metamorphic rocks. It is most commonly a disseminated accessory mineral in all of these rocks.

Concentrated masses of magnetite are produced by magmatic segregation (liquid immiscibility?)

in some mafic/basic gabbros. Massive replacement deposits of magnetite are produced by

contact metamorphism of calcareous metamorphic rocks. Sedimentary iron formations and

banded iron formations often contain substantial amounts of magnetite. These large igneous,

metamorphic and sedimentary bodies constitute most of the concentrated magnetite deposits that

are mined commercially. Magnetite is also concentrated in the heavy mineral population of

detrital sediments.

Uses: Magnetite is a major ore of iron (Fe), the primary material used in the manufacture of a

wide range of steel alloys for the manufacture of tools, appliances, automobiles, trucks, ships,

weapons, bridges and building supports. It is also used in the production of cast iron for

cookware, pipes and furnaces and of wrought iron for fences and patio furniture. Iron is widely

used as a dietary supplement, especially for people with symptoms of anemia.

MALACHITE:

Composition: Cu2CO3)(OH)2; a hydrous carbonate mineral

Crystal System: Monoclinic (2/m): a = 9.48Ǻ, b = 12.03Ǻ, c = d3.21Ǻ; α = 90o, β = 99

o, γ =

90o

Crystal Habit: rare individual crystals are short prismatic to acicular; commonly as acicular to

capillary crystals in radiating, banded, colloform (botryoidal) aggregates; also as drusy

encrustations and granular to earthy masses


Hardness: 3½ -4


Cleavage/Fracture: 2 orientations of prismatic cleavage not at 90o; perfect, but rarely

visible; a third orientation of fair cleavage is also not generally discernible;


Colors: various shades of green; often intense, bright green

Streak: pale, but distinctly green

Luster: vitreous, even adamantine; silky in fibrous-radiating aggregates

Other: effervesces readily in cold, dilute hydrochloric acid (HCl); commonly alters to

azurite

Diagnostic Characteristics: The bright green color and ready effervescence in HCl are

diagnostic; the radiating, banded botryoidal habit is useful



Relief: moderately high to very high positive

Color/Pleochroism: green; pleochroic from nearly colorless, pale green, yellow-green,

blue-green

Birefringence: 0.254; extreme; the faded colors (“creamy white”) masked by green color

Optic Sign: biaxial (–); 2Vx = 40-45o (moderate)\

Diagnostic Properties: the combination of pleochroism in green shades, common

acicular-radiating-banded colloform habit, high relief, extreme birefringence and biaxial

(-) optics with moderate 2V are characteristic; the association with other oxidized zone

minerals is also helpful

Occurrences/Associations: Malachite most commonly occurs in the oxidized zone of

hydrothermal vein and replacement deposits that contain copper-bearing sulfide minerals such as

chalcopyrite and bornite which are oxidized by descending, acidic (CO2-rich) meteoric water. It

is typically associated with azurite, cuprite, limonite (goethite), smithsonite and cerussite, among

others.

Uses: a minor ore of copper; large masses are prized by collectors as decorative pieces; banded

malachite is widely used as a stone in jewelry; when powdered, malachite provides green

pigments

MANGANITE:

Composition: MnO(OH);


o, γ = 90

o;

so pseudo-orthorhombic

Crystal Habit: slender prismatic to stubby prismatic or columnar crystals, with striated faces; in

divergent aggregates; also in massive, granular aggregates, as encrustations and as a constituent

of mixtures of manganese minerals called wad


Hardness: 4; moderate


Cleavage/Fracture: one orientation of pinacoidal cleavage {010}; perfect

Diaphaneity: opaque

Colors: dark gray to black

Streak: dark brown to brownish black

Luster: metallic

Other: twins common; frequently alters to pyrolusite and other manganese minerals

Diagnostic Properties: Recognized chiefly by its striated prismatic crystals, black color,

brownish streak and hardness of 4; pyrolusite is softer; romanechite is harder; there are

many related manganese oxides and hydroxides, many of which occur in finely

crystalline mixtures called wad

Microscopic Properties: opaque

Occurrences/Associations: Manganite, forms in low temperature hydrothermal veins where it is

associated with hausmanite, barite, calcite and siderite. It is also a common constituent of wad,

finely crystalline mixtures of multiple manganese oxide, hydroxide and oxyhydroxide minerals

that form by the alteration of manganese bearing minerals in soils, by precipitation on ocean

floors (“manganese nodules”) and in bogs (bog manganese) and also accumulate in a variety of

other near surface environments.

Uses: Manganite is an ore of manganese (Mn) which is primarily used as a hardening agent in

the manufacture of steel for support beams, automobiles and machinery. It is also used as a

hardening agent with aluminum and copper, e.g., in the production of electrical wires and

transmission lines. Manganese is also use as a coloring agent in the production of glassware and

ceramics. It is used in the production of potassium permanganate (KMnO4), used as a to kill

bacteria and algae in water and wastewater treatment.

MARIALITE: see SCAPOLITE

MEIONITE: see SCAPOLITE

MARCASITE:

Composition: FeS2; polymorphic with pyrite


90o, γ = 90

o

Crystal Habit: individual crystals are typically tabular, less commonly prismatic; commonly

occurs in radiated and colloform (reniform to globular) aggregates, often with a drusy surface


Hardness: 6 – 6 ½; hard


Cleavage/Fracture: two orientations not 90o; fair, generally not discernable

Diaphaneity: opaque

Colors: pale bronze yellow (almost white)

Streak: black

Luster: metallic

Other: commonly alters to limonite which often forms pseudomorphs after marcasite

Diagnostic Properties: Distinguished from pyrite which it closely resembles by lighter

(whiter) color on fresh surfaces and different crystal forms

Microscopic Properties: opaque; pleochroic creamy white to yellowish with pinkish and

greenish tints in reflected light

Occurrences/Associations: Most marcasite occurs in low temperature (telethermal and

epithermal) hydrothermal vein and replacement deposits where it is associated with galena,

sphalerite, fluorite, barite and calcite.

Uses: Marcasite has no significant value, though some sulfur for sulfuric acid and some iron are

extracted from it during the processing of ore minerals with which it is associated.

MICROCLINE:

Composition: KAlSi3O8; a tectosilicate mineral; a feldspar; polymorphic with orthoclase and

sanidine; significant sodium (Na+1

) may substitute for potassium (K+1

) at the time of formation,

but much sodium (Na+1

) exsolves to form albite during cooling, so microcline is commonly

perthitic or microperthitic; microcline typically possesses a high degree of Al-Si ordering


o, γ = 88

o

Crystal Habit: stubby prismatic to flattened prismatic-tabular crystals; commonly in coarse,

granular aggregates and disseminated crystals


Hardness: 6; hard


Cleavage/Fracture: 2 cleavage orientations at very nearly right angles (89.4 and 90.6o);

one perfect; one good to very good


Colors: variable; typically white, salmon pink-red or yellowish; green to blue-green

variety is called amazonite

Streak: white

Luster: vitreous

Other: commonly perthitic, with blebs, stringers and/or patches of intergrown albite;

commonly alters to sericite and clay minerals

Diagnostic Properties: distinguished as a potassium feldspar by its hardness, colors,

commonly perthitic nature and two sets of cleavage near right angles; sanidine is

generally more transparent; sanidine and orthoclase are rarely blue green and crystallize

in the monoclinic system, but cleavage fragments of orthoclase can otherwise be

indistinguishable from microcline without optical microscopy




Color/Pleochroism: colorless; often “dusty” or “cloudy” from alteration to clay minerals

Birefringence: 0.006 – 0.007; low; maximum colors are first order grays and white

Twinning: tapered polysynthetic pericline twins; often in gridiron (scotch plaid or tartan)

pattern with polysynthetic albite twins; also paired Carlsbad twins; microperthitic

intergrowths common as well

Optic Sign: Biaxial (–); 2Vx = 50 – 85o; moderate to very high 2V; generally high to

very high; increases with degree of order

Other: commonly perthitic, with blebs, stringers and/or patches of intergrown albite

Diagnostic Properties: the combination of low negative relief, “cloudiness”, two sets of

right angle cleavage, low birefringence and “gridiron” or “scotch plaid” twinning are

diagnostic; microperthitic intergrowths and the biaxial negative character with relatively

high 2V help to confirm the identification; orthoclase lacks tapered pericline twins and

albite twins, so never displays gridiron twinning; sanidine is less cloudy and possesses a

smaller 2V

Occurrences/Associations: Microcline is an abundant mineral in alkali-rich, deep seated

plutonic igneous rocks including pegmatite, granodiorite, granite and alkali granite, and syenite

and alkali syenite that formed under conditions of slow cooling. It is also common in

quartzofeldspathic and pelitic metamorphic rocks including gneiss and schist formed in the

amphibolite and granulite facies. It is also a significant component of detrital sedimentary rocks

including arkosic sandstone and also occurs as authigenic cement in sedimentary rocks.

Uses: As an important rock forming mineral, especially in granitoids, microcline as a constituent

of such rocks finds use as a dimension stone for countertops, wall and building facings and

monuments. Ground up and mixed with clays and quartz, it also finds use in the manufacture of

glass and ceramics products including tiles, plumbing fixtures and electrical insulators and as

filler in paints, plastics and paper products. Amazonite is cut and polished for use as an

ornamental stone.

MOLYBDENITE:

Composition: MoS2; a sulfide mineral


90o, γ = 120

o

Crystal Habit: typically as platy to tabular crystals with hexagonal outlines; less commonly as

stubby hexagonal prisms; occurs in massive, foliated or scaly aggregates


Hardness: 1 – 1½; very soft


Cleavage/Fracture: one orientation of perfect basal pinacoidal {0001} cleavage

Diaphaneity: opaque

Colors: bluish silver gray

Streak: gray black, often with a slight greenish tint

Luster: metallic

Other: greasy feel typical of very soft minerals; sectile; plates are flexible, but not elastic

Diagnostic Properties: Can be distinguished from graphite which is equally soft by

its much higher specific gravity, its slightly bluish gray color and the slightly greenish

tint to the gray black streak

Microscopic Properties: Opaque; white, with a bluish tint, in reflected light

Occurrences/Associations: Molybdenite most commonly occurs in high temperature

hydrothermal (hypothermal) veins and disseminations in granitoid igneous rocks including

aplites and pegmatites. It also occurs in contact metamorphic rocks such as skarn. Associated

minerals include quartz, calcite, fluorite, chalcopyrite, pyrite, cassiterite, scheelite and

wolframite.

Uses: Molybdenite is the major ore of molybdenum (Mo) which is used in the production of high

strength steels. It also finds use in lubricants and, as an essential trace element, in dietary

supplements and fertilizers.

MONAZITE:

Composition: (Ce,Ln,Y,Th)PO4; a rare earth phosphate mineral; any rare earth element can

substitute in the REE site, but cerium and lanthanum are the most common; actinides, especially

thorium, but also uranium, can substitute; some silicate (SiO4) can substitute for phosphate (PO4)


o, γ = 90

o

Crystal Habit: roughly equant to bladed or stubby prismatic crystals; most crystals small, except

in some pegmatites; typically as disseminated crystals


Hardness: 5; upper end of moderate


Cleavage/Fracture: generally not discernible; a single fair cleavage orientation exists


Colors: typically reddish brown to yellow

Streak: white

Luster: subvitreous; resinous to waxy

Other: commonly radioactive due to the thorium (and lesser uranium) content

Diagnostic Characteristics: in scarce larger crystals, monazite can be recognized by its

radioactivity, its high specific gravity which distinguishes it from titanite (sphene) and

its hardness that distinguishes it from zircon



with thorium content

Relief: very high positive

Color/Pleochroism: colorless to pale yellow or yellow brown; pleochroism not generally

visible

Birefringence: 0.045 – 0.075; very high; colors up to 3rd

and 4th

order, with some

“fading”

Twinning: simple twins common

Optic Sign: Biaxial (+); 2Vz = 6 – 19o; small

Diagnostic Properties: the very high relief, colorless to pale yellow color, very high

birefringence, monoclinic crystals and biaxial (+) optics with a large 2V are

characteristic; titanite is typically pale brown, possesses higher refractive indices, relief

and birefringence, typically occurs in “wedge-shaped” crystals and possesses a larger 2V;

zircon and xenotime are tetragonal and uniaxial and xenotime tends to be pleochroic in

thin-section, zircon colorless

Occurrences/Associations: Monazite is typically disseminated as an accessory mineral in

igneous alkali-rich granitoids, granitic pegmatites, syenites, alkali syenites and carbonatites. It

also occurs as an accessory mineral in metamorphic rocks such as marbles, gneisses and

granulites. Monazite is very stable in the weathering environment and so also occurs

concentrated with other resistant heavy minerals (magnetite, ilmenite, zircon, rutile and

tourmaline), in placer deposits in sediments and sedimentary rocks.

Uses: Monazite is an important ore of rare earth elements and Thorium. Rare earth elements are

widely used in modern manufactured products. For example, cerium is used in catalytic

converters; lanthanum is used in nickel hydride batteries for hybrid cars, in catalytic converters

and in mantles for gas lamps; and yttrium is used in computer monitors, TV screens and

fluorescent lights. Thorium is used in incandescent lights, the mantles for gas lamps and as a

fuel in nuclear reactors. Several rare earth elements are used in the manufacture of high

refractive index glass and in an increasing variety of other applications.

MONTMORILLONITE: see SMECTITE

MUSCOVITE:

Composition: KAl2AlSi3O10(OH)2; a phyllosilicate mineral; a mica group mineral with a T-O-T

structure with interlayer cations; there is a limited substitution solid solution series with

paragonite [NaAl2AlSi3O10(OH)2] in which sodium (Na+1

) substitutes for potassium (K+1

) in

amounts limited by size difference to about 10%; microscopic crystals of muscovite or

paragonite are commonly called sericite; in all of these, fluorine (F-1

) commonly substitutes for

hydroxyl ion (OH-1

); chlorine (Cl-1

) does so less commonly; lepidolite[K(LiAl)2-

3AlSi3O10(OH)2] is a scarce, lithium (Li+1

) mica, closely related to muscovite; many other small-

scale substitutions occur in all of these micas


o, γ = 90

o;

Crystal Habit: tabular (six-sided cross sections) to platy-scaly crystals; common in foliated



Hardness: on cleavage surfaces: 2½; soft; perpendicular to cleavage ~4; moderate

Specific Gravity: 2.8 -2.9; moderate

Cleavage/Fracture: one set of basal pinacoidal {001} cleavage; perfect


Colors: colorless to pale yellow or greenish; also pale gray, pale brown or reddish

Streak: white

Luster: vitreous

Other: flexible and elastic in thin sheets

Diagnostic Properties: the light color, along with the low hardness, tendency to split

into thin flakes or sheets along one perfect cleavage and the elasticity of such sheets are

diagnostic; paragonite is similar; lepidolite typically is shade of pink or lilac owing to its

elevated lithium content, but can be indistinguishable from muscovite in hand samples.



Relief: low to moderate, positive; changes somewhat with rotation


Birefringence: 0.036 – 0.049; high; striking colors are upper second to third order

Twinning: usually not discernable

Optic Sign: Biaxial (–); 2Vx = 28 – 47o; low to moderate 2V; paragonite possesses a

lower 2Vx = 0 – 40o and the 2Vx of lepidolite ranges between 0 – 60

o

Other: “birdseye” maple appearance near extinction which occurs at small angles (0 –

3o) to the prominent cleavage; length slow parallel to cleavage, but difficult to see

because of deep interference colors; wavy extinction in bent flakes; hexagonal sections

display symmetrical extinction;

Diagnostic Properties: the combination of a single perfect cleavage, lack of color,

birdseye extinction, near parallel extinction and biaxial (–) optics with a small to

moderate 2V are characteristic; distinguished from biotite by biotite’s strong colors and

pleochroism; most phlogopite is also pleochroic with paler colors than biotite, but darker

than muscovite, a significantly smaller 2V and a generally different set of associations;

lepidolite and paragonite are similar to muscovite in that they are colorless, but generally,

being nearly uniaxial, possess a smaller 2V; lepidolite possesses lower birefringence and

occurs almost exclusive in lithium-rich pegmatite and paragonite is common only in

metamorphic rocks; distinctions between these three minerals may require XRD and

other analytical techniques

Occurrences/Associations: Muscovite is an abundant mineral in igneous, metamorphic and

sedimentary rocks. In igneous rocks, it is especially abundant in peraluminous granitoids,

pegmatites and aplites and other felsic rocks. It is also common, in the form of sericite, as the

product of hydrothermal alteration of feldspars and other aluminum-bearing minerals in such

rocks. Muscovite occurs widely in low- to fairly high-grade pelitic metamorphic rocks including

slate, phyllite, schist, and gneiss. Because it is fairly resistant to decomposition during

weathering, muscovite is a common constituent of many detrital sediments and sedimentary

rocks, especially arkosic sandstone. Paragonite occurs widely in low-grade metamorphic rocks

including slate, phyllite and schist. Lepidolite occurs chiefly in lithium-rich pegmatite where it

is associated with beryl, tourmaline, spodumene, amblygonite, albite-rich plagioclase and

potassium feldspar.

Uses: Sheets of muscovite are used in the “windows” of microwave tubes and industrial

furnaces. Ground muscovite is used in the production of capacitors and transistors for use in

electronic devices, in the production of insulation, paints and plastics and in the manufacture of

wall paper with a high sheen. Consumer products that use muscovite include lipstick, nail

polish, eye shadow and “glitter”.

NATROLITE:

Composition: NaAl2Si3O10•2H2O; a tectosilicate mineral; a zeolite mineral; some substitution

of potassium (K+1

) and sodium (Ca+2

) for calcium (Na+1

) commonly occurs; the latter is balanced

by substitution of aluminum (Al+3

) silicon (Si+4

)

Crystal System: Orthorhombic (2mm): a = 18.27Å, b = 18.61Å, c = 6.59Å; α = 90o, β = 90

o, γ

= 90o

Crystal Habit: occurs as slender prismatic to acicular crystals; commonly in radiating

aggregates; also fibrous and granular aggregates; finely crystalline massive aggregates and as

disseminated crystals also occur




Cleavage/Fracture: two orientations of cleavage near right angles; perfect; but not

always discernible in aggregates


Colors: typically colorless to white; less commonly shades of gray, blue, yellow and pink

Streak: white

Luster: vitreous

Diagnostic Properties: The light color, hardness and slender prismatic to acicular

crystals in radiating to fibrous aggregates are distinctive; finely crystalline aggregates

generally require XRD analysis for accurate identification



Relief: low-moderate to moderate, negative


Birefringence: 0.012 – 0.013; low; first order yellow to orange colors

Other: fibers are length slow

Optic Sign: Biaxial (+); 2Vz = 0 – 64o; very low to moderately high 2V

Other: longitudinal sections through crystals are length slow


zeolite minerals; stilbite is similar, but is biaxial (–) and typically possesses a sheaf-like

habit rather than a radial-fibrous one; heulandite typically lacks the fibrous-radial habit,

has slightly lower birefringence and typically a lower 2V; chabazite is pseudo-

rhombohedral, with 3 sets of pseudocubic cleavage; laumontite is biaxial (–) with three

sets of cleavage; analcime is isotropic

Occurrences/Associations: Natrolite is a relatively widespread zeolite mineral. Coarse crystals

are especially common in cavities, including vesicles, in mafic/basic rocks such as basalt where

they are associated with calcite, analcime, chabazite, heulandite, stilbite, thomsonite and

pectolite. Less commonly, these occur in intermediate volcanic rocks such as andesite.

Natrolite is also reported from alkaline soils formed in desert environments and from some

contact metamorphic aureoles.

Uses: Natrolite, like most natural and synthetic zeolites, is used, after heating and dehydration, to

produce materials that remove water vapor from carbon dioxide, from refrigerants such as Freon

and from many organic chemicals used in industrial applications. It is also used as a molecular

sieve used to separate molecules of different sizes as in separating nitrogen from air to produce

nearly pure oxygen gas. Because zeolites can selectively absorb selected molecules, they are

widely used in applications that include reducing the hardness of water for cleaner laundry by the




NEPHELINE:

Composition: (Na0.75,K0.25)AlSiO4; a tectosilicate mineral; a feldspathoid mineral; somewhat

higher amounts of potassium (K+1

) may substitute for sodium (Na+1

); a related, non-isomorphous

mineral is kalsilite (KAlSiO4)

Crystal System: Hexagonal (6) a1 = 9.99Å, a2 = 9.99Å, c = 8.37Å; α = 90o, β = 120

o, γ = 90

o

Crystal Habit: individual crystals are stubby hexagonal prismatic; more commonly in granular

aggregates or as disseminated crystals




Cleavage/Fracture: not discernible; conchoidal fracture


Colors: typically white to gray; occasionally with a pale yellow or pale green tint

Streak: white

Luster: typically subvitreous to greasy

Other: commonly alters to cancranite (often yellow), sodalite (blue or gray), calcite,

clay minerals and analcime

Diagnostic Properties: resembles some milky-smoky quartz, but is softer, has a

characteristic greasy luster and different alteration products



Relief: low, negative; less commonly very low positive

Color/Pleochroism: colorless; may be pale gray or “cloudy”

Birefringence: 0.003 – 0.005; Low; first order grays

Twinning: occur, but rarely discernable


Other: rectangular longitudinal sections are length fast and display parallel extinction;

may display zoning

Diagnostic Properties: the combination of low relief, low birefringence, uniaxial (–)

optics and lack of recognizable cleavage are characteristic; quartz has low, but positive

relief, is generally less “cloudy” and is uniaxial (+); orthoclase possesses 2 sets of right

angle cleavage and commonly displays paired Carlsbad twins and/or is perthitic; the

distinction from kalsilite requires advanced chemical analysis or XRD

Occurrences/Associations: Nepheline occurs widely in alkali-rich, silica undersaturated

plutonic and volcanic igneous rocks including foid syenite, foidite and phonolite, where it is

associated with sodalite, cancranite, leucite, albite-rich plagioclase, potassium feldspars, aegerine

and aegerine-augite, and riebeckite and arfvedsonite.

Uses: The principle use of nepheline is as a constituent in the manufacture of glass and ceramic

ware. In a few places, nepheline is mined as a source of aluminum ore.

NICKELINE: aka Nickelite

Composition: NiAs; an arsenide mineral

Crystal System: Hexagonal (6/m2/m2/m): a1 = 3.60Ǻ, a2 = 3.60Ǻ, c = 5.01Ǻ; α = 120o, β =90

o,

γ = 120o

Crystal Habit: rare, tabular hexagonal crystals; typically in massive aggregates, but also in

colloform (reniform to stalactitic) aggregates


Hardness: 5 - 5½; hard (moderately so)


Cleavage/Fracture: none; subconchoidal fracture

Diaphaneity: opaque

Colors: typically pale copper red; dark gray to black tarnish

Streak: pale brownish black


Diagnostic Properties: the pale copper red color is distinctive; the high specific gravity,

brownish black streak and tarnish are helpful indicators

Microscopic Properties: Opaque; highly reflective in reflected light in pleochroic shades of

yellow-pink to brownish pink

Occurrences/Associations: Nickeline is a scarce mineral that most frequently occurs in

mafic/basic and ultramafic/ultrabasic igneous rocks, where it is formed by magmatic

segregations and associated with pentlandite, pyrrhotite, chalcopyrite and pyrite.

Uses: Nickeline is a minor ore of nickel used to strengthen steel alloys for the production of

machines. It is also an essential ingredient in stainless steel used in “silverware” and kitchen

appliances and in a number of other alloys. Nickel is also used with copper in so-called nickel

coins.

NITER: (also known as “saltpeter”)

Composition: KNO3; a nitrate mineral; closely related to nitratite (NaNO3)


90o, γ = 90

o

Crystal Habit: commonly as acicular crystals in radiating to fibrous aggregates


Hardness: 2; soft


Cleavage/Fracture: two orientations of prismatic cleavage {011} not at right angles;

good


Colors: white

Streak: white

Luster: vitreous to silky

Other: extremely soluble

Diagnostic Properties: the “cooling” taste is diagnostic; the habit and low hardness are

also helpful


Refractive Indices: nα =1.332; nβ = 1.504; nγ = 1.504

Relief: low to very low, negative; sections in which both na and nβ or nγ are visible

produce a “twinkling” effect as the relief changes when stage is rotated under plane light


Birefringence: 0.176; very high to extreme; so high order, faded colors (“creamy white”)

in some orientations

Twinning: simple pairs common

Optic Sign: Biaxial (–); 2Vx = 5 -12o; very low 2V

Diagnostic Properties: the lack of color, negative relief, “twinkling” effect, extreme

birefringence, and biaxial (–) optics with very low 2V are characteristic of this rare

mineral which is associated with other continental “evaporites”

Occurrences/Associations: Niter and nitratite occur as surface encrustations and as precipitates

in saline soils in areas with very low rainfall and are associated with gypsum, halite and other

evaporate minerals.

Use: Niter and nitratite are mined as a source of nitrate for fertilizers.

OLIGOCLASE: see PLAGIOCLASE

OLIVINE:

Composition: (Mg,Fe)2SiO4; a nesosilicate (orthosilicate) mineral; a complete solid solution

series exists between forsterite (Mg2SiO4) and fayalite (Fe2SiO4); forsterite-rich olivine is much

more abundant than fayalite-rich olivine; manganese (Mn+2

) commonly substitutes, especially

for iron (Fe+2

) in small amounts; forsterite-rich olivine is stable at higher temperatures than

fayalite-rich olivine; montecellite is a scarce, calcium rich olivine (CaMgSiO4)

Crystal System: Orthorhombic (2/m2/m2/m): forsterite: a = 4.75Å, b = 10.20Å, c = 5.98Å; α

= 90o, β = 90

o, γ = 90

o; fayalite: a = 4.82Å, b = 10.48Å, c = 6.09Å; α = 90

o, β = 90

o, γ = 90

o

Crystal Habit: individual crystals tabular to bladed or equant; typically in massive or granular

aggregates or as disseminated crystals


Hardness: 6½ – 7; hard

Specific Gravity: 3.22 – 4.39; moderate to high; increasing with forsterite and therefore

iron (Fe+2

) content

Cleavage/Fracture: not generally discernible; conchoidal fracture


Colors: commonly pale to light olive green to yellowish green; becoming darker as

fayalite component and therefore iron (Fe+2

) content increases; fayalite-rich olivine is

medium green or brown

Streak: white

Luster: vitreous

Other: crystals sometimes zoned with cores relatively enriched in forsterite component

and rims enriched in fayalite component; alters to reddish- to yellow-brown “iddingsite”

and to orange to greenish “chlorophaeite” which are isotropic or nearly isotropic mixtures

of alteration products

Diagnostic Properties: The pale green, olive green, yellow green color, high

translucency, conchoidal fracture, hardness and granular to massive habit are diagnostic.


Refractive Indices: nα = 1.636 – 1.827; nβ = 1.651 – 1.868; nγ = 1.669 – 1.879; increases

with fayalite, therefore iron (Fe+2

) content; maximum forsteritic olivine (>Fo50) indices

are in the range nα = 1.730; nβ = 1.760; nγ = 1.770

Relief: moderate to very high; increases with fayalite, therefore iron (Fe+2

) content

Color/Pleochroism: typically colorless; fayalite-rich olivine may display slight

pleochroism in shades of pale yellow color, greenish yellow and amber

Birefringence: 0.033 – 0.052; high to very high; increase somewhat with fayalite,

therefore iron (Fe+2

) content; maximum colors from higher second order to third order;

maximum birefringence for forsteritic olivine is approximately 0.042

Twinning: none reported

Optic Sign: Biaxial (+); 2Vz = 84 – 90o; very high 2V, for forsterite-rich (>Fo87) olivine;

Biaxial (–) with 2Vx = 74 – 90o, for other compositions (Fo87 – Fo50) and 46 – 74

o for

fayalitic (<Fo50) olivine

Other: commonly alters to serpentine and/or iddingsite (a reddish brown to golden

brown mixture of limonite and/or hematite and clay minerals)

Diagnostic Properties: forsteritic olivine is recognized by its lack of color, its moderate

to high relief, its high birefringence and its general lack of cleavage; optically positive

olivine and olivine with very high 2V (>75o) is forsteritic; fayalite is often lightly colored

in shades of yellow-brown-green, has higher indices, relief and birefringence and is

optically negative with a smaller 2V; the pyroxenes with which forsteritic olivine is

commonly associated possess 2 sets of near right angle cleavage

Occurrences/Associations: Forsterite-rich olivine is the most abundant mineral in

ultramafic/ultrabasic rocks in the upper mantle (it inverts to spinel below 400km depth). In such

rocks, including dunite and peridotite, it is associated with orthopyroxene, clinopyroxene and

spinel-group minerals and with sparse amphibole and phlogopite. Forsterite-rich olivine is also

common in mafic/basic igneous rocks such as olivine gabbro, troctolite and basalt where it is

associated with calcic plagioclase, orthopyroxene, and clinopyroxene. Rare fayalite-rich olivine

occurs in iron-rich rocks, including granitoids. Forsterite-rich olivine is also a common mineral

in medium- to high-grade metamorphic rocks. It is especially common in calcareous, calcsilicate

rocks where it is associated with grossularite garnet, calcite, dolomite, diopside, tremolite,

phlogopite, epidote, and wollastonite, and in mafic and ultramafic metamorphic rocks where it is

associated with orthopyroxene, clinopyroxene, calcite plagioclase and garnet. Montecellite is a

scarce component of calcareous contact metamorphic rocks and of alkalic mafic-ultramafic

igneous rocks such as feldapathoidal peridotite and basalt and lamprophyre. Because it

decomposes rapidly during weathering, olivine is rare in detrital sediments.

Uses: Deep green, transparent specimens of olivine called peridote are used as gems

(birthstones). Olivine is used to make refractory sand for castings in foundries and for refractory

bricks and is sometimes used in abrasive products.

OMPHACITE:

Composition: (Ca,Na)(Al,Mg,Fe)Si2O6; single-chain inosilicate; a clinopyroxene with a range

of compositions between jadeite (which contains more sodium and aluminum) and augite (which

contains more calcium, iron and magnesium) and diopside (which contains more calcium and

magnesium)

Crystal System: Monoclinic (2/m): a = 9.45 – 9.68Å, b = 8.57 – 8.90Å, c = 5.23 – 5.28Å; α =

90o, β = 105 - 108

o, γ = 90

o

Crystal Habit: scarce crystals are stubby prismatic, with squarish four- to eight-sided crystals

(one or two sets of prisms); commonly anhedral; as disseminated crystals and massive to

granular aggregates




Cleavage/Fracture: two orientations of prismatic {210} cleavage near right angles (87o

and 93o); a prominent parting that bisects the cleavage may be discernible


Colors: typically bright green to dark green

Streak: gray to gray-green

Luster: vitreous to satiny

Other: alters to fibrous green amphiboles; retrograde metamorphism produces

breakdown into diopside and plagioclase intergrowths

Diagnostic Properties: omphacite cannot be reliably distinguished from many other

pyroxenes, but the bright green color and association with garnet is suggestive;

distinguished from green amphiboles by the near right angle cleavage




Color/Pleochroism: colorless to pale green

Birefringence: 0.012 – 0.028; low to moderate; maximum colors from first order yellow

to second order reds and yellow


Optic Sign: Biaxial (+); 2Vz = 56 – 84o; moderate to high

Other: cross-sections (showing near right angle cleavage) display symmetrical extinction


cleavage near right angles and squarish 4- 8-sided cross sections; omphacite is quite

similar to both diopside and augite in color, but diopside and augite possess generally

higher birefringence, higher refractive indices and relief and typically have a smaller 2V;

omphacite is closely similar to jadeite, but occurs in the eclogite facies rather than in the

blueschist facies so that the associations are somewhat different

Occurrences/Associations: Omphacite is a high pressure clinopyroxene. It is an essential

constituent (with garnet) of most eclogite formed by the metamorphism of mafic/basic igneous

rocks under very high pressure and temperature conditions in the eclogite facies at depths that

exceed 40 – 50km. Omphacite also occurs in regional metamorphic rocks of the high

temperature/high pressure granulite (granulite II) facies and in high P/T rocks of the blueschist

facies. It always suggests deep burial of the rocks in which it occurs.

Uses: Omphacite has no commercial value.

OPAL:

Composition: SiO2•nH2O; a silica group, amorphous mineraloid, lacking a long-range crystal

structure, in which silica spheres are packed together, with various amounts of water that range

from about 4% to 20% in the interstices between them

Crystal System: none; since amorphous

Crystal Habit: there are no crystals; but opal generally occurs in irregular masses


Hardness: 5½ - 6½; hard, but less so than other silica group species

Specific Gravity: 2.0 -2.3; low

Cleavage/Fracture: no cleavage; excellent conchoidal fracture


Colors: highly variable; white, yellow, brown, red, green, gray and blue

Streak: white

Luster: typically waxy to resinous

Other: often opalescent with significant play of colors, as in “fire opal”

Diagnostic Properties: The conchoidal fracture and waxy to resinous luster are

indicative; possesses a lower hardness than microcrystalline silica such as chert and

chalcedony, although the latter can possess a waxy luster; the play of colors and isotropic

nature are clinchers


Refractive Indices: 1.43 – 1.46; low

Relief: moderate-high; negative

Color/Pleochroism: colorless; may be pale gray or pale brown

Birefringence: none, isotropic

Optic Sign: none, isotropic

Diagnostic Properties: the combination of moderate-high negative relief, lack of color

or pale color, lack of cleavage, conchoidal fracture and isotropic optics are

characteristic; most volcanic glass possesses a higher refractive index and lower negative

relief; analcime commonly occurs as trapezohedral crystals, sodalite may occur as

dodecahedral crystals and both minerals possess somewhat higher refractive indices

an lower negative relief; XRD may be necessary to sort these out in finely crystalline

examples

Occurrences/Associations: Opal occurs widely in surface environments and in sedimentary

rocks. It is a component of siliceous sinter in hot springs deposits, of low temperature

hydrothermal vein deposits and of much petrified wood. It is the primary component of the shells

of diatoms that accumulate in many aqueous environments to form diatomite or diatomaceous

earth and of radiolaria that accumulate on deep ocean floors. It replaces calcareous shells. Opal

also fills voids in volcanic rocks and is a significant cement in some detrital sediments,

especially those of volcanoclastic origin.

Uses: Opals of various colors are significant gemstones, widely used in jewelry. Organic opal,

in the form of diatomite or diatomaceous earth, is widely used as a soil conditioner, in filtration

agents for removing impurities and in insulation products.

ORTHOCLASE:

Composition: KAlSi3O8; a tectosilicate mineral; a feldspar; polymorphic with microcline and

sanidine; much sodium (Na+1


) at the time of formation, but

much sodium (Na+1

) exsolves to form albite during cooling, so orthoclase is commonly perthitic

or microperthitic; commonly has an intermediate degree of Al-Si ordering between well-ordered

microcline and more disordered sanidine


o, γ =

90o

Crystal Habit: stubby prismatic to flattened prismatic crystals; commonly in coarse, granular

aggregates and disseminated crystals


Hardness: 6; hard


Cleavage/Fracture: 2 cleavage orientations at right angles; one perfect; one good to very

good


Colors: variable; typically white, gray, salmon pink or yellowish

Streak: white

Luster: vitreous

Other: sometimes perthitic, with blebs, stringers and/or patches of intergrown albite, but

less often than microcline; commonly alters to sericite and clay minerals

Diagnostic Properties: distinguished as a potassium feldspar by its hardness, colors,

sometimes perthitic nature and two sets of cleavage near right angles; sanidine is

generally more transparent; microcline is more commonly perthitic, crystallizes in the

triclinic system and is sometimes blue-green, but cleavage fragments of orthoclase can

otherwise be indistinguishable from microcline without optical microscopy




Color/Pleochroism: colorless; often “dusty” or “cloudy” from alteration to clay minerals


Twinning: paired Carlsbad penetration twins common; also simple, paired Mannebach

and Baveno twins; microperthitic intergrowths are common as well; lacks polysynthetic

pericline and albite twins and therefore the gridiron (scotch plaid or tartan) twinning

characteristic of microcline

Optic Sign: Biaxial (–); 2Vx = 40 – 75o; moderate to high 2V; increases with degree of

order and with sodium (Na+1

) content

Other: commonly perthitic; graphic and granophyric intergrowths with quartz common;

also as blebs and stringers in plagioclase (antiperthite); alters to sericite and clay

minerals; replaced by calcite, albite and quartz

Diagnostic Properties: the combination of low negative relief, “cloudiness”, low

birefringence, biaxial (–) optics with a sizeable 2V and common, simple paired twins are

characteristic; sanidine is similar, but generally less cloudy, possesses a significantly

smaller 2V and occurs mostly in volcanic-hypabyssal igneous rocks; microcline typically

possesses polysynthetic, tapered pericline twins and albite twins in a gridiron pattern

under crossed polars; untwinned orthoclase resembles quartz, but the latter is uniaxial

(+) and is generally less “cloudy” except in hydrothermal veins

Occurrences/Associations: Orthoclase is an important rock forming mineral. It is abundant in

alkali-rich plutonic igneous rocks including pegmatite, granodiorite, granite and alkali granite,

syenite and alkali syenite, especially those formed at moderate cooling rates at moderate depth.

It is also common in quartzofeldspathic and pelitic metamorphic rocks including gneiss and

schist formed in the greenschist and amphibolite facies and in contact metamorphic aureoles. It

is also a significant component of detrital sedimentary rocks including arkosic sandstone and

occurs as authigenic cement in sedimentary rocks.

Uses: As an important rock forming mineral, especially in granitoids, orthoclase finds use as a

dimension stone for countertops, wall and building facings and monuments. Ground up and

mixed with clays and quartz, it also finds use in the manufacture of glass and ceramics products

including tiles, plumbing fixtures and electrical insulators and as filler in paints, plastics and

paper products.

ORTHOPYROXENE: (ENSTATITE-HYPERSTHENE)

Composition: (MgFe)2Si2O6; single-chain inosilicate; an orthorhombic pyroxene solid solution

series exists between end member components enstatite (Mg2Si2O6) and ferrosilite (Fe2Si2O6),

with several intermediate compositions including hypersthene (MgFeSi2O6); orthopyroxenes

can be subdivided on the basis of percent enstatite end-member (En) into: enstatite (En88-100),

bronzite, often considered a variety of hypersthene (En70-88), hypersthene (En50-70),

ferrohypersthene (En30-50), eulite (En12-30) and ferrosilite (En0-12); the is rare in nature;

significant substitution of other cations occurs; calcium (Ca+2

) substitutes significantly at high

temperatures and exsolves to form separate phases during cooling that produce exsolution

lamellae of calcium-rich clinopyroxene

Crystal System: Orthorhombic (2/m2/m2/m): a = 18.22 – 18.43Å, b = 8.81 – 9.08Å, c = 5.17

– 5.24Å; α = 90o, β = 90

o, γ = 90

o

Crystal Habit: crystals are usually stubby prismatic with squarish four- or eight-sided cross

sections; more commonly as massive to granular aggregates or as disseminated crystals; acicular-

capillary, radial fibrous aggregates also occur



Specific Gravity: 3.21 – 3.96; moderate to moderate to high; increasing with ferrosilite,

therefore iron (Fe+2

) content

Cleavage/Fracture: two orientations of prismatic {210} cleavage near 90o (88

o and 92

o);

good only; a prominent parting commonly bisects the cleavages

Diaphaneity: nearly transparent to barely translucent; less transparent with increasing

iron (Fe+2

) content

Colors: enstatite-rich specimens are usually clove to honey brown or greenish brown;

ferrosilite-rich specimens of hypersthene and ferrohypershene are medium to dark green,

brown or greenish black


Luster: vitreous to pearly; submetallic on some bronzite

Other: prominent parting bisect the cleavage in many specimens

Diagnostic Properties: the two nearly right angle cleavages distinguish orthopyroxene

from amphiboles of similar color; the clove to honey brown or greenish brown color and

generally greater transparency distinguish enstatite-rich orthopyroxene from most

clinopyroxene; the submetallic luster of bronzite is diagnostic; hypersthene resembles

many clinopyroxenes and requires a thin-section or advanced analytical technique for

proper identification



Relief: moderately high to high, positive

Color/Pleochroism: enstatite is colorless; hypersthene is colorless to pleochroic in

shades of brownish pink, pink, yellow, red, brownish red, greenish yellow, green, gray

green and blue green

Birefringence: 0.007 – 0.020; low to moderate, increasing with iron (Fe+2

) content;

maximum first order gray through first order reds is typical

Twinning: none; but exsolution lamellae of clinopyroxene are common and mechanical

twins can occur

Optic Sign: enstatite (En88-100): Biaxial (+), 2Vz = 48 – 90o; moderate to very high

bronzite, hypersthene, ferrohyperthene, eulite (En12-88): Biaxial (–), 2Vx = 50 – 90o;

moderate to very high; ferrosilite is Biaxial (+), but rare in nature

Other: 4-8 sided cross-sections display two sets of cleavage near right angles and

symmetrical extinction; longitudinal sections display parallel extinction; hypersthene

(and bronzite) that formed at depth, commonly contains fine exsolution lamellae of

exsolved calcic pyroxene such as augite or diopside; mechanical twin lamellae have

similar looking appearance; poikilitic olivine common;

Diagnostic Properties: the combination of 4-8 sided cross-sections and two sets of

cleavage near right angles distinguish orthopyroxene from orthoamphiboles such as

anthophyllite; the parallel extinction and relatively low birefringence distinguish

orthopyroxene from most clinopyroxenes which also possess angular extinction; enstatite

is biaxial (+), whereas other orthopyroxenes are biaxial (–)

Occurrences/Associations: Magnesium-rich orthopyroxene such as enstatite and bronzite are

abundant in mafic/basic igneous rocks such as norite, gabbro and basalt and in

ultramafic/ultrabasic rocks including peridotite (harzburgite and lherzolite) and pyroxenite. It is

commonly associated with olivine, clinopyroxenes, spinels, and, in the case of mafic rocks, with

calcic plagioclase. Orthopyroxenes are also abundant in relatively high-grade metamorphic

rocks. For example, enstatite is abundant in regionally metamorphosed ultramafic rocks of the

granulite facies and iron-rich eulite is a common component of metamorphose banded iron

formations (BIF). Orthopyroxene is also a major constituent of the “stony” component of

meteorites formed in the asteroid belt.

Uses: Orthopyroxene is of limited economic value. Some specimens find use as a semi-precious

gemstone and orthopyroxenite is used as a dimension stone for counter tops.

PARAGONITE: see MUSCOVITE

PECTOLITE:

Composition: Ca2NaH(SiO3)3; single-chain inosilicate; a pyroxenoid mineral with rotated

single chains; small amounts of manganese (Mn+2

) may substitute for calcium (Ca+2

)


o, γ = 102

o

Crystal Habit: individual crystals are acicular to capillary; commonly occurs in radiating

aggregates of such crystals; less commonly occurs in massive aggregates (like alabaster gypsum)


Hardness: 4 ½ – 5; moderate to hard


Cleavage/Fracture: two orientations of pinacoidal cleavage {100} and {001} near 90o

(85o and 95

o); perfect; not discernible in acicular to capillary crystals with radiating habit


Colors: typically white to creamy white

Streak: white

Luster: silky in radiating aggregates; dullish in compact massive aggregates

Diagnostic Properties: the radiating acicular to capillary crystals are characteristic;

resembles wollastonite, but the latter has a different occurrence in medium-high grade

metamorphic rocks rather than as a cavity filling in volcanic rocks; the zeolite mineral

natrolite is similar, but usually forms divergent aggregates of slender prismatic crystals

rather than radiating aggregates


Refractive Indices:

Relief: moderate to moderate plus, positive


Birefringence: 0.026 – 0.035; moderate to high; maximum colors are middle to upper

second order

Twinning: rare

Optic Sign: Biaxial (+); 2Vz = 50– 63o; moderate to moderately high 2V

Other: fibers are length slow, extinction angles are < 10o

Diagnostic Properties: the combination of radial-fibrous habit, length slow crystals,

moderate-high relief and birefringence and biaxial (+) optics with moderate-large 2V are

characteristic; wollastonite can be similar, but its fibers are length fast, its extinction

angle higher, its birefringence lower and its occurrence and association completely

different; tremolite is biaxial (–) with a larger 2V, its 2 cleavages at 56o and 124

o are

often visible and it also has a completely different occurrence and association

Occurrences/Associations: Pectolite occurs as a secondary, hydrothermal mineral in cavities

and vesicles in volcanic rocks such as basalt and andesite. It is commonly associated with

calcite, prehnite and zeolite minerals that include stilbite, heulandite, chabazite, natrolite and

analcime. Pectolite is a scarce mineral in serpentinites and is a primary mineral in some contact

metamorphic skarns and rare foid syenite and phonolite.

Uses: Pectolite has no major uses.

PENTLANDITE:

Composition: (Fe,Ni)9S8; a sulfide mineral

Crystal System: Isometric (4/mbar32/m): a = 9.93Ǻ, b = 9.93Ǻ, c = 9.93Ǻ; α =90o, β = 90

o, γ

= 90o

Crystal Habit: crystals rare; most commonly in massive to granular aggregates


Hardness: 3½ - 4

Specific Gravity: 4.6 – 5.0; increasing with nickel content

Cleavage/Fracture: none; irregular fracture; does have octahedral {111} parting in

rare larger crystals

Diaphaneity: opaque

Colors: light bronze yellow

Streak: pale bronze brown

Luster: metallic

Diagnostic Properties: pentlandite can be distinguished from pyrrhotite by its lack

of magnetism and from pyrite by is bronze color, streak and inferior hardness

Microscopic Properties: opaque; whitish in reflected light with a creamy pinkish or brownish

tint

Occurrences/Associations: Most pentlandite occurs in mafic/basic and ultramafic/ultrabasic

igneous rocks, where it is formed by magmatic segregations and is associated with nickeline,

pyrrhotite, chalcopyrite and pyrite.

Uses: Pentlandite is the major ore of nickel used to strengthen steel alloys for the production of

machines. It is also an essential ingredient in stainless steel used in “silverware” and kitchen

appliances and in a number of other alloys. Nickel is also used with copper in so-called nickel

coins.

PHLOGOPITE: see BIOTITE

PIEDMONTITE: see EPIDOTE

PIGEONITE:

Composition: (Fe,Mg,Ca)2Si2O6; single-chain inosilicate; a clinopyroxene; the proportions of

magnesium and iron vary greatly; the molecular proportion of magnesium (Mg+2

) + iron (Fe+2

)

is always much greater than that of calcium (Ca+2

= 10 – 30%); displays extensive solid solution,

especially at high temperatures, with augite [(Ca,Na)(Mg,Fe,Fe,Al,Ti)Si2O6] which has much

more calcium (Ca+2

), more sodium (Na+1

) and aluminum (Al+3

) and less magnesium (Mg+2

) than

pigeonite


o, γ = 90

o

Crystal Habit: rare crystals are stubby prismatic to prismatic {210} with squarish four- to eight-

sided cross sections; commonly in granular aggregates or coarse cleavable aggregates or as



Hardness: 6; hard



o);



Colors: typically greenish brown to brown; brownish black when iron-rich

Streak: gray


Other: exsolution lamellae of more calcic plagioclase (augite) are common; alters to

chlorite, biotite, limonite, hematite and clay minerals

Diagnostic Properties: pigeonite cannot be distinguished in hand specimen from other

clinopyroxenes or orthopyroxene with similar colors; if discernable, the crude, nearly

right angle cleavage and/or four- to eight-sided squarish crystals distinguish it from

amphiboles with similar color


Refractive Indices: nα = 1.682 – 1.732; nβ = 1.684– 1.732; nγ = 1.705 – 1.757


Color/Pleochroism: colorless, pale shades of greenish brown, pink, brownish pink,

yellow, green, and reddish brown; very slightly pleochroic

Birefringence: 0.023 – 0.029; moderate; maximum second order green to second order

red colors


Optic Sign: Biaxial (+); 2Vz = 0 - 32o; low to very low 2V

Other: squarish, 4-8 cross-sections (showing near right angle cleavage) display

symmetrical extinction; longitudinal sections are length slow; maximum extinction

angles are 39 - 52o; phenocrysts are commonly zoned


cleavage near right angles distinguish pigeonite from most amphiboles; augite is

similar, but has a higher 2V (> 45o) and often lacks the pinkish colors that characterized

pigeonite, in addition to green and brown; orthopyroxenes show parallel extinction in all

longitudinal sections; olivine lacks cleavage has much higher birefringence and generally

higher 2V; iron-rich orthopyroxene (hypersthene) and iron-rich olivine are also

optically (–)

Occurrences/Associations: Pigeonite is a comparatively uncommon clinopyroxene which

occurs primarily in volcanic and shallow intrusive (hypabyssal) igneous rocks, especially as

small phenocrysts and as a groundmass constituent. It occurs in rocks in mafic/basic and

intermediate rocks including basalt, diabase, gabbro, andesite, diorite and some dacite. It often

inverts to orthopyroxene with augite exsolution lamellae during long-term cooling.

Uses: Pigeonite has no commercial value. It is a constituent of some dimension stone used for

countertops.

PLAGIOCLASE:

Composition: (Na,Ca)(Si,Al)AlSi2O8; a tectosilicate mineral; a feldspar group mineral;

complete coupled solid solution series between albite (Ab) = NaAlSi3O8 with large ion sites

occupied by sodium (Na+1

) and ¾ of the tetrahedral sites occupied by silicon (Si+4

) and anorthite

(An) = CaAl2Si2O8 with large ion sites occupied by Ca and ½ of the tetrahedral sites occupied by

aluminum (see chapter 3 for detailed discussion). Plagioclase varieties are distinguished by the

percentage of anorthite (An) end member as follows: Albite (<An10), Oligoclase (An10-30),

Andesine (An30-50), Labradorite (An50-70), Bytownite (An70-90) and Anorthite (>An90); both

high-temperature (relatively disordered) and low-temperature (relatively ordered) plagioclases

occur, with transitional ordering common; chemical zoning is very common in plagioclase,

typically with cores enriched in anorthite relative to rims, but also with oscillatory or reverse

zoning; a complete solid solution series exists at high temperatures between albite (NaAl3Si3O8)

and potassium feldspar (KAl3Si3O8), but at lower temperatures exsolution occurs to produce

perthite (blebs, stringers or patches of albite in a potassium feldspar host) or antiperthite (blebs,

stringers or patches of potassium feldspar in an albite host); anorthoclase is a sodic feldspar

with substantial potassium feldspar in solid solution (Ab65Or35 to Ab90Or10) that occurs in

volcanic rocks and has fine-scaled gridiron twinning analogous to that in microcline;

Crystal System: Triclinic (ī): a = 8.14 – 8.18Å, b = 12.79 – 12.88Å, c = 7.158 – 14.17Å; α = 93

– 94o, β = 116 – 117

o, γ = 88 – 92

o

Crystal Habit: individual crystals are tabular to bladed; may even be platy; typically in coarsely

crystalline granular or finely crystalline massive aggregates or as disseminated crystals



Specific Gravity: 2.6 – 2.8; moderate; increases with anorthite content

Cleavage/Fracture: two orientations of cleavage near right angles (86 – 87o and 93 –

94o); one perfect the, other very good


Colors: variable, typically white to light to dark gray; also pale green, yellow or blue

Streak: white

Luster: vitreous to pearly or opalescent

Other: twinning striations common on some cleavage surfaces; alters to sericite, clay

minerals, calcite, epidote group minerals and/or zeolite group minerals

Diagnostic Properties: The hardness and near right angle cleavage are indicative of

feldspar; distinguished from potassium feldspar by the presence of fine striations on the

basal cleavage surface that results from albite twinning; the salmon pink color also

distinguishes potassium feldspar as do the common macroscopic perthitic intergrowths;

to complicate matters, many metamorphic plagioclase crystals are untwinned


Refractive Indices: nα = 1.527 – 1.577; nβ = 1.531 – 1.585; nγ = 1.534 – 1.590,

increasing with anorthite content; the boundary between albite and oligoclase occurs at

approximately nα = 1.533; nβ = 1.537; nγ = 1.542, so most albite has negative relief;

between oligoclase and andesine at approximately nα = 1.543; nβ = 1.547; nγ = 1.552;

between andesine and labradorite at approximately nα = 1.554; nβ = 1.558; nγ = 1.562;

between labradorite and bytownite at approximately nα = 1.563; nβ = 1.568; nγ = 1.573;

between bytownite and labradorite at approximately nα = 1.572; nβ = 1.578; nγ = 1.583

Relief: low negative for most albite, low positive for the others with positive relief

increasing with anorthite content

Color/Pleochroism: colorless; typically “dusty” or “cloudy” due to alteration to clays

and sericite

Birefringence: 0.007 – 0.013; low; increasing with anorthite content; maximum colors

are first order grays and white to yellow (in very calcic plagioclase)

Twinning: polysynthetic albite twins are very common; they tend to be very narrow in

oligoclase, a little wider in albite and andesine and can be quite wide in bytownite and

anorthite; paired Carlsbad twins occur with albite twins, especially in more calcic

plagioclase; tapered polysynthetic pericline twins occur alone or in combination with the

other two; they are generally at an oblique angle to albite twins; rarer twin types occur;

some metamorphic plagioclase is untwinned

Optic Sign: Biaxial (–) and Biaxial (+) depending on composition and ordering; mostly

biaxial negative with 2Vx = 45 – 90o; also biaxial positive with 2Vz = 58 – 90

o; high to

very high 2V; albite and labradorite are (+), andesite and anorthite are (–) and oligoclase

and bytownite may be (+) or (–)

Other: zoning is common, especially in hypabyssal and volcanic rocks; normal zoning

is characterized by progressive enrichment in albite (lower-temperature) component from

core to rim as might be expected from continuous cooling; reverse zoning is the

opposite, sodic cores with more calcic rims, recording changes in equilibrium

compositions driven by changes in vapor pressure or magma mixing; oscillatory zoning

displays fluctuations from core to rim with calcic core to sodic zones to calcic zones and

sodic zones common, recording fluctuations in equilibrium composition; intergrowths

are common, including (1) perthite: intergrowths of plagioclase in K-feldspar, (2)

antiperthite: intergrowths of K-feldspar in plagioclase, (3) myrmekite: bleb or worm-

like intergrowths of quartz in plagioclase

Diagnostic Properties: the combination of lack of color, “cloudiness”, low relief, low

birefringence, two sets of cleavage near right angles, albite (and other) twinning and

biaxial optics are characteristic; orthoclase is similar, but lacks polysynthetic albite twins

and has lower refractive indices and negative relief; microcline typically possesses

gridiron twins and also has lower refractive indices and negative relief; sanidine lacks

albite twins, is less cloudy and has a smaller 2V; quartz is generally less “cloudy” and

lacks cleavage; the combination of refractive index, optic sign and occurrence/association

is helpful in sorting out likely subvarieties; many optical methods, most of which use

extinction angles and twinning, are available for the determination of plagioclase

composition using thin-sections—consult a good optical mineralogy book for details;

XRD and advanced chemical analytical techniques are required for truly accurate

determinations of An content

Occurrences/Associations: Plagioclase is the most abundant rock forming mineral in Earth’s

crust. Albite is common in alkali-rich igneous rocks including pegmatite, alkali granite and

granite, alkali syenite and syenite and their aphanitic equivalents such as rhyolite and trachyte.

Albite is also common in low temperature pelitic, mafic and quartzofeldspathic metamorphic

rocks formed in the albite epidote hornfels, prehnite-pumpellyite, greenschist and even blueschist

facies. It is also fairly common in detrital sedimentary rocks and as an authigenic mineral in

sedimentary rocks. Oligoclase is common in igneous rocks such as granite, granodiorite,

monzonite and their volcanic equivalents such as rhyodacite, dacite and latite. It is reasonably

common in detrital sedimentary rocks. Andesine is common in intermediate igneous rocks such

as diorite, monzodiorite and andesite. Labradorite is abundant in mafic igneous rocks including

gabbro and basalt and in related rocks such as norite and anorthosite. Bytownite is relatively

rare, occurring primarily in mafic igneous rocks such as gabbro and norite and occasionally in

ultramafic rocks such as websterite and troctolite. Anorthite is rare, occurring most commonly

in contact metamorphic skarns and related calcsilicate rocks. None of the calcium-rich

plagioclases are common in sedimentary rocks because, their chemical instability during

weathering means they tend to be decomposed before they can be dispersed from the source area.

Uses: As a major rock forming mineral in granitoids, diorite and gabbro, plagioclase is an

important constituent of dimension stone used for countertops, wall and building facings,

headstones and sculptures. Albite, ground up and mixed with clays and quartz, finds use in the

manufacture of glass and ceramics products including tiles, plumbing fixtures and electrical

insulators and all plagioclase finds use as a filler in paints, plastics and paper products.

PLATINUM:

Composition: Pt; a native element (native metal); commonly alloyed with iron (Fe), and

platinum group metals such as palladium (Pd), rhenium (Rh), osmium (Os) and iridium (Ir), as

well as copper (Cu), gold (Au) and nickel (Ni).


o, γ

= 90o

Crystal Habit: scaly grains; rare, equant, malformed cubic crystals; typically in small; typically

disseminated, but small, irregular masses do occur


Hardness: 4-4½; moderate

Specific Gravity: 21.4; but usually impure, so lower (14-19) when alloyed with less

dense metals


Diaphaneity: opaque

Colors: steel gray to dark gray

Streak: whitish steel gray

Luster: metallic

Other: can be magnetic when alloyed with iron

Microscopic Properties: Opaque; bluish-white in reflected light

Occurrences/Associations: Most platinum deposits are associated with large, layered gabbroic

intrusions (LLGI’s), especially with the, ultramafic layers of such intrusions where it is

associated with olivine, pyroxene, chromite and magnetite. It also occurs as placer deposits in

detrital sedimentary rocks in close proximity to ultramafic source rocks.

Use: Native platinum is the major ore of platinum (Pt), a rare metal. The major use of platinum is

as a catalyst in chemical reactions, for example in automobile catalytic converters used to control

exhaust emissions. Platinum is highly refractory and resistant to chemical attack and so is used

in a variety of high-temperature applications. Platinum is also used in jewelry and some

electrical equipment.

POTASH FELDSPAR: see MICROCLINE, ORTHOCLASE and SANIDINE

PREHNITE:

Composition: Ca2Al(AlSi3O10)(OH)2; a phyllosilicate mineral; a small percentage of iron (Fe+3

)

commonly substitutes for aluminum (Al+3

)

Crystal System: Orthorhombic (mm2): a = 4.65Å, b = 4.49Å, c = 18.52Å; α = 90o, β = 90

o, γ

= 90o

Crystal Habit: individual crystals are platy to tabular; commonly occur in radiating to fan-

shaped, banded, colloform (globular) aggregates, often with platy drusy surfaces; commonly

amygdaloidal; double fans yield an hourglass aggregate



Specific Gravity: 2.8 – 2.9; moderate; increases with iron (Fe+3

) content

Cleavage/Fracture: one basal pinacoidal {001} cleavage orientation; good; commonly

not discernable


Colors: typically light to medium green; also white, gray, yellow, pink

Streak: white


Other: alters to chlorite or zeolite minerals

Diagnostic Properties: the green color, single cleavage and hardness, in combination

with the radiating, globular habit are quite distinctive; smithsonite is softer and

effervesces in HCl; hemimorphite possesses a much higher specific gravity



with iron content

Relief: moderate; higher for iron-rich varieties


Birefringence: 0.020 – 0.035; moderate; increases with iron (Fe+3

) content; may be

anomalous; maximum colors from first order red to second order blue; abnormal

interference colors may occur

Twinning: uncommon, but both lamellar and sector twins occur

Optic Sign: Biaxial (+); 2Vz = 64 – 70o; Large 2V

Diagnostic Properties: the combination of crystal habit, lack of color, moderate +

refractive indices and relief, moderate + birefringence and biaxial (+) optics with high 2V

are characteristic; specimens with hourglass structure and/or anomalous birefringence are

distinctive; zeolite minerals have moderate negative relief

Occurrences/Associations: Prehnite commonly occurs as a cavity filling mineral in basic/mafic

to intermediate volcanic rocks where it fills veins or occurs as amygdules in association with

zeolite minerals, pectolite and calcite. It is also an abundant constituent of low-grade

metamorphic rocks of the zeolite and prehnite-pumpellyite facies where it occurs with

pumpellyite, lawsonite, albite-rich plagioclase, epidote, chlorite and zeolites. It is a less common

constituent of some skarns where it occurs with calcite, dolomite and a variety of calcsilicate

minerals. It also forms by the alteration of plagioclase in igneous rocks.

Uses: Prehnite is sometimes used as a decorative or ornamental stone.

PUMPELLYITE:

Composition: Ca2MgAl2(SiO4)(Si2O7)(OH)2•H2O; a sorosilicate mineral; significant variations

in composition occur due to large amounts of the usual substitutions (Na+1

for Ca+2

) (Fe+2

, Fe+3

and Al+3

for Mg+2

)(Fe+3

for Al+3

) which define different end members of the pumpellyite group


o, γ = 90

o

Crystal Habit: individual crystals commonly slender prismatic or bladed to acicular; commonly

in radiating to crudely fibrous aggregates, in massive aggregates or as disseminated crystals


Hardness: 5 - 6; hard

Specific Gravity: 3.16 – 3.25; moderate; increases with iron (Fe) content

Cleavage/Fracture: one basal {001} cleavage; good; not easily discerned

Diaphaneity: nearly transparent to translucent

Colors: variable; typically green to bluish green: also colorless, brown, or greenish black

Streak: white or pale gray

Luster: vitreous

Diagnostic Properties: Difficult to distinguish from members of the epidote group from

which it can be distinguished by XRD or other analytical techniques



Relief: moderate-high, positive

Color/Pleochroism: colorless to pale green, blue-green, green, yellow-green, yellow,

yellow brown; distinctly pleochroic

Birefringence: 0.008 - 0.020; low to moderate; maximum colors are first order gray to

red-purple

Optic Sign: Biaxial (+), most commonly, or (–); 2Vz = 7 – 90o; 2Vx = 70 – 90

o; very low

to very high 2V

Other: maximum extinction angles with respect to cleavage 24o – 45

o;

Diagnostic Properties: the combination of pleochroism in pale shades of green-yellow-

brown, high relief, relatively low-moderate birefringence, and biaxial optics is

characteristic; resembles epidote group minerals; epidote generally has smaller

extinction angles, mottled birefringence, slightly higher birefringence, commonly

displays anomalous interference colors, is biaxial (–) and never has a small-moderate 2V;

clinozoisite can be quite similar to pumpellyite, but can be distinguished by the common

anomalous birefringence and often larger extinction angles; zoisite is orthorhombic, so

possesses parallel extinction;

Occurrences/Associations: Pumpellyite most commonly occurs in the low grade prehnite-

pumpellyite facies (transitional from zeolite facies to higher grades) and in the blueschist facies

metamorphic rocks, formed by moderate to high pressure and low temperature (high P/T)

conditions associated with rocks being rapidly forced downward along Franciscan trajectories to

moderate to high pressures (6-12kbar), while remaining relatively cool (200-400oC), during

subduction in trench-arc systems. In the prehnite-pumpellyite facies, pumpellyite common

occurs as amygdules in mafic volcanic rocks or as a product of the low pressure, low temperature

hydrothermal metamorphism of such rocks where it is commonly associated with prehnite,

epidote, zeolites, calcite and albite-rich plagioclase. In the blueschist facies it is associated with

lawsonite, muscovite, kyanite, glaucophane, chlorite, actinolite, zoisite, aragonite and albite-rich

plagioclase in pelitic, mafic, and rare calcareous rocks formed under prehnite-pumpellyite and

blueschist facies conditions.

Uses: Pumpellyite has no economic value.

PYRITE:

Composition: FeS2; polymorphic with marcasite

Crystal System: Isometric (2/mbar3): a = 5.42Ǻ, b = 5.42Ǻ, c = 5.42Ǻ; α = 90o, β = 90

o, γ =

90o

Crystal Habit: equant individual striated cubes {001} and pyritohedra {102} are common;

octahedra {111} less so; very common is massive to granular aggregates and as disseminated

crystals; also as acicular crystals with in fibrous or stellated aggregates


Hardness: 6 – 6½


Cleavage/Fracture: none discernable; one indistinct cleavage exists

Diaphaneity: opaque

Colors: pale brass yellow, tarnishes dark to iridescent

Streak: greenish to brownish black

Luster: metallic

Other: penetration twins yield “iron crosses”

Diagnostic Properties: superior hardness, brittleness and black streak distinguish pyrite

from gold; superior hardness, lighter color and crystal forms distinguish pyrite from

chalcopyrite; superior hardness and less bronze-hued color distinguish it from

pyrrhotite; similar to marcasite, but has a somewhat deeper, less white color and

generally different crystal forms

Microscopic Properties: opaque; creamy white to yellow white in reflected light; not pleochroic

Occurrences/Associations: Pyrite is widely distributed in igneous, sedimentary and

metamorphic rocks and in hydrothermal vein and replacement deposits. It is the most abundant

sulfide mineral in hydrothermal deposits that form under all temperatures from telethermal to

hypothermal. It is an abundant accessory mineral in igneous rocks of all compositions and in

some mafic rocks concentrated in large segregations. It is quite common in sedimentary rocks,

especially in coal, black shale and related mudrocks formed under reducing conditions. It is also

a common accessory in many metamorphic rocks.

Uses: Despite its abundance, pyrite does not have any real economic value. Both iron and sulfur

are extracted from pyrite during the processing of ore minerals that occur with it, but pyrite is not

mined for its own value.

PYROLUSITE: see WAD

PYROPE: see GARNET

PYROXENE: see varieties, including AEGERINE, AUGITE, DIOPSIDE, HEDENBERGITE,

JADEITE, OMPHACITE, \ORTHOPYROXENE, PIGEONITE, SPODUMENE

PYROPHYLLITE:

Composition: Al2Si4O10(OH)2; a phyllosilicate mineral with a T-O-T structure; rather small

amounts of substitution occur: aluminum (Al+3

) for silicon (Si+4

), magnesium (Mg+2

), ferrous

iron (Fe+2

), ferric iron (Fe+3


); some interlayer sodium (Na+1

), potassium

(K+1

) and/or calcium (Ca+2

) may occur


o, γ =

90o; some pyrophyllite crystallizes in the triclinic system with similar dimensions for the a- and

b-axes, but roughly half for the c-axis as the unit cell is one T-O-T layer thick, rather than two,

and with α = 91o

Crystal Habit: individual crystals are scaly to tabular, less commonly acicular; occur most

commonly in foliated crystal aggregates; also radiating, fibrous and massive aggregates


Hardness: 1 – 2; very soft


Cleavage/Fracture: one orientation of basal pinacoidal {001} cleavage; perfect; not

generally discernible in finely crystalline aggregates


Colors: typically white; also pale shades of yellow, apple green, gray, blue or brown

Streak: white

Luster: typically pearly; may be dull in some finely crystalline aggregates

Other: thin sheets are flexible, but not elastic; soft enough to mark colored paper and to

possess a greasy feel; sectile

Diagnostic Properties: the combination of very low hardness, greasy feel and luster,

pale colors and foliated habit distinguish pyrophyllite from all minerals except talc;

muscovite has a vitreous luster and is elastic in thin sheets; pyrophyllite’s occurrence in

pelitic metamorphic and hydrothermal associations rather than magnesian metamorphic

and hydrothermal associations permits it to be distinguished from talc


Refractive Indices: nα = 1.552 – l.556; nβ = 1.586 – 1.589; nγ = 1.596 – 1.601



Birefringence: 0.043 – 0.045; high; maximum third order colors common

Twinning: rare; poorly developed

Optic Sign: Biaxial (–); 2Vx = 53 – 62o; moderate to high

Other: cleavage traces are length slow; elongate crystals are length fast

Diagnostic Properties: the combination of lack of color, low-moderate relief, high

birefringence, biaxial (–) sign with large 2V, and crystal habit are distinctive; resembles

talc, but talc possesses a significantly smaller 2V (5o – 30

o); may resemble some sericite

and kaolinite, but sericite has near parallel extinction and lower birefringence and

kaolinite has much lower birefringence.

Occurrences/Associations: Pyrophyllite occurs primarily in relatively low- to medium-grade,

relatively high P/T pelitic metamorphic rocks and is commonly associated with kyanite. Some

large deposits of pyrophyllite may be metabauxite deposits. Pyrophyllite also forms as a product

of the hydrothermal alteration of aluminum-rich rocks that contain kyanite, sillimanite,

andalusite, corundum, topaz, muscovite and/or alkali feldspars.

Use: Pyrophyllite’s primary use is in the production of refractory tiles to line kilns and furnaces.

It is also used in the manufacture of floor and wall tiles, pottery and porcelain products. It is also

used with gypsum in the manufacture of sheetrock wall board. Pyrophyllite has long been used

to make carved ornamental pieces.

PYRRHOTITE:

Composition: Fe1-XS; a sulfide mineral; some of the Fe+2

ions are missing from the structure to

produce omission defects; their charges are balance by the substitution of two Fe+3

ions in other

sites for every three Fe+2

omissions; copper, nickel and cobalt may substitute as well

Crystal System: Monoclinic (2/m); a = 6.87Ǻ, b = 11.90Ǻ, c = 22.72Ǻ; α = 90o, β = 90.1

o, γ =

90o; so almost orthorhombic in low-temperature examples; is hexagonal (6/m2/m2/m) in high-

temperature examples

Crystal Habit: tabular hexagonal crystals most common, with inverted structures; very common

as massive to granular aggregates and as disseminated crystals


Hardness: 3.5 – 4.5; moderate


Cleavage/Fracture: none; uneven/irregular to subconchoidal fracture

Diaphaneity: opaque

Colors: pale bronze-yellow color; may have brownish tint; tarnishes black or

iridescent

Streak: dark gray to black


Other: Ferrimagnetic (like magnetite, but somewhat less strongly)

Diagnostic Properties: distinguished from pyrite by magnetism, softer hardness and

color which is more bronze in tint, from chalcopyrite by color and crystal form and from

pentlandite by slightly deeper color, magnetism and darker streak

Microscopic Properties: opaque; light brown color in reflected light

Occurrences/Associations: Pyrrhotite most commonly occurs in mafic/basic and

ultramafic/ultrabasic igneous rocks, where it is associated with pentlandite, nickeline,

chalcopyrite and pyrite. It is also fairly common in high temperature hydrothermal vein and

replacement deposits, with chalcopyrite, galena, sphalerite and arsenopyrite, and in contact

metamorphic aureoles.

Uses: Pyrrhotite is not an especially valuable mineral. It is mined for its nickel, cobalt, copper

and platinum where they substitute in sufficient quantities, as at Sudbury, Ontario.

QUARTZ: see CHERT and CHALCEDONY for microcrystalline varieties


sodium (Na+1

), potassium (K+1

) and (Ca+2



) for silicon (Si+4

)occurs in two polytypes: rhombohedral α –

quartz (low quartz) and hexagonal β – quartz (high quartz), the former being the stable form in

relatively low-moderate temperature (< 573oC) and low-moderate pressure (< 22Kbar = 2.2GPa)

environments under which most rocks in Earth’s crust form; β – quartz (high quartz) inverts

readily through non-displacive transformation into α – quartz (low quartz) on cooling; the

characteristics of α – quartz (low quartz) are therefore cited below; many varieties of

macrocrystalline quartz exist, based primarily on color, including rock crystal (colorless, quite

transparent), milky quartz (white), rose quartz (pink), smoky quartz (brown to nearly black),

amethyst (purple to violet) and citrine (yellowish); quartz is polymorphic with other silica

group minerals including tridymite and cristobalite; rarer polymorphs include the high pressure

silica minerals coesite and stichovite which occur in meteorite impactites and likely in the deep

mantle; microcrystalline varieties of quartz are dealt with separately under chert and chalcedony

Crystal System: Hexagonal (32): a1 = 4.91Å, a2 = 4.91Å, c = 5.41Å; α = 90o, β = 120

o, γ = 90

o

Crystal Habit: typically as slender to stubby prismatic crystals, striated perpendicular to prism

axis, with six-sided (two sets of trigonal prisms) cross sections and rhombohedral terminations

(two sets of which may resemble hexagonal pyramids); common in divergent crystal aggregates

and as drusy coatings; very common as more or less equant crystals and grains in coarsely

crystalline, granular aggregates and as disseminated crystals and grains


Hardness: 7; hard to very hard


Cleavage/Fracture: none; conchoidal fracture, especially evident in large crystals


Colors: highly variable, often gray; colorless in rock crystal, white in milky quartz,

yellow in citrine, purple-violet in amethyst, pink in rose quartz and brown to black in

smoky quartz; mineral inclusions may also produce distinctive color as in green (chrome

mica) or red (hematite) aventurine

Streak: white


Other: because of its low symmetry, quartz is strongly piezoelectric and pyroelectric

Diagnostic Properties: the combination of hardness, conchoidal fracture, vitreous

luster and often distinctive coloring and crystal habit are diagnostic; garnet is harder and

has isometric crystals; cordierite resembles quartz, but is typically a bluish-gray color and

less transparent





Birefringence: 0.009; low; maximum first order grays and white; yellow only if thin-

section is too thick

Twinning: penetration twins are common


Other: basal sections remain dark on rotation under crossed polars and yield excellent

centered optic axis figures; euhedral crystals are length slow with parallel extinction;

strained crystals may possess a small 2V and typically display undulatory extinction (in

which different sector go to extinction at different times as the stage is rotated, the

extinction shadows “sweeping across” the crystal; intergrowths with feldspars are

common and include (1) graphic (and micrographic) intergrowths of optically aligned

quartz in K-feldspar; (2) granophyre: intergrowths of plumose blebs of quartz in K-spar

and (3) myrkmekite: vermicular (worm-like) intergrowths of quartz and plagioclase;

inclusions of acicular rutile, tourmaline, and apatite and of chlorite and fluid droplets are

also common

Diagnostic Properties: the combination of lack of color, low relief, low birefringence, lack of

alteration and frequent clarity, lack of cleavage, lack of twinning, parallel extinction and length

slow character and uniaxial (+) optics are diagnostic; other colorless minerals with low relief and

low birefringence possess different properties; nepheline has negative relief, more alteration and

is uniaxial (–); untwinned orthoclase has negative relief, two cleavages near right angles and is

biaxial (–) with a substantial 2V; beryl has higher relief and is uniaxial (–); cordierite is generally

more altered, commonly twinned and is biaxial (–) with a substantial 2V; some scapolite

resembles quartz, but it possesses cleavage and is uniaxial (–)

Occurrences/Associations: Macrocrystalline quartz is an abundant mineral in igneous,

metamorphic and sedimentary rocks. In igneous rocks, it occurs as an essential mineral in silica-

oversaturated rocks that include pegmatite, granite, granodiorite, tonalite and as phenocrysts in

the volcanic equivalents rhyolite, rhyodacite and dacite. It is much less common in silica-

oversaturated syenite, monzonite, diorite, gabbro and the volcanic equivalents trachyte, latite,

andesite and basalt. In metamorphic rocks, quartz is the primary mineral in quartzite, is

abundant in many schists and gneisses, and is a significant component of some slate, phyllite,

granulite and hornfels. Because it is stable over a wide range of metamorphic conditions, quartz

occurs in rocks of most of the major metamorphic facies. Quartz is quite resistant to

decomposition in most surface environments and is therefore selectively concentrated in detrital

sedimentary rocks where it is the major constituent of quartzarente sandstone and a significant

constituent of most arkose, most siltstone and many litharentites and mudstones.

Macrocrystalline quartz is also a significant cement in such rocks and forms by the replacement

of carbonate material during diagenesis. Macrocrystalline quartz is also an abundant mineral in

hydrothermal vein and replacement deposits.

Use: Macrocrystalline quartz has a wide variety of uses. Quartz is widely mined from relatively

pure quartzarenite sand, sandstone and quartzite deposits for use as a source of silica for the

manufacture of glass. It is also mined from such deposits as a source of elemental silicon for use

in the manufacture of semiconductors used in the electronics and computers. Silicon is also used

to produce silicone used in the production of silicone caulking to seal fixtures in home and for

breast implants. Quartz sand and gravel is widely used in aggregates used in construction

including concrete, asphalt and mortar. Increasingly, clean quartz sand is being used in

hydrofracturing black shale layers to obtain natural gas. Their pyroelectric and piezoelectric

properties permit “pure” quartz crystals (now mostly synthetic) to be used in the production of

electronic oscillators and pressure gauges. Macrocrystalline quartz is widely used as a

semiprecious gemstone with fine examples of amethyst, citrine, rose quartz, smoky quartz and

aventurine used in rings, pendants, earrings and necklaces. Rock crystal is highly prized by

many “new age” spiritualists for its supposed mystical and healing properties.

RHODOCHROSITE:

Composition: MnCO3; a carbonate; isostructural with calcite, magnesite, siderite and

smithsonite, all members of the rhombohedral (calcite) group of carbonate minerals; forms a

complete solid solutions series with siderite (as Mn and Fe substitute for each other) and a

limited substitution series with magnesite, smithsonite, and even calcite

Crystal System: Hexagonal (Rhombohedral (bar32/m): a1 = 4.78Ǻ, a2 = 4.78Ǻ, c = 15.66Ǻ; α

= 120o, β = 90

o, γ = 120

o

Crystal Habit: rare as equant, rhombohedral {10bar10} crystals with curved faces similar to

dolomite and siderite; common in massive, coarsely crystalline aggregates, granular aggregates

or encrustations


Hardness: 3 ½ - 4

Specific Gravity: 3.7 – 3.8; moderately high, especially with increasing iron content

Cleavage/Fracture: 3 orientations at 90o

(73o and 107


perfect

Diaphaneity: quite translucent

Colors: typically shades of pink to rose red; may also be shades of gray or brown

Streak: white

Luster: vitreous

Other: effervesces readily in HCl only if powdered or if acid is heated

Diagnostic properties: The pink to rose red color and rhombohedral cleavage are

characteristic; rhodonite lacks cleavage and is harder (H = 6)



Relief: variable; from low-moderate to very high positive; sections in which both nω and

nε are visible produce a “twinkling” effect as the relief changes when stage is rotated

under plane light

Color/Pleochroism: colorless to pale pink (look closely!); may be slightly pleochroic

Birefringence: 0.219; extreme; so high order, faded colors (“creamy white”) in most

orientations

Twinning: relatively rare rhombohedral twins





uniaxial (–) optics are characteristic of rhombohedral carbonates such as rhodochrosite;

rhodochrosite is distinguished from other rhombohedral carbonates, except siderite, by its

very high maximum refractive indices and relief and its consistently positive relief in all

orientations; siderite is often brownish (from oxidation of iron) under plane light and

displays yellow brown to brown colors in hand specimen, rather than the pink-rose red

colors shown by rhodochrosite

Occurrences/Associations: Rhodochrosite occurs primarily in low-medium temperature

hydrothermal veins and replacement deposits with sulfide minerals such as galena, pyrite,

tetrahedrite, chalcopyrite and bornite. Rhodonite, barite, calcite, fluorite and quartz are other

common minerals in such deposits.

Uses: Pink to rose red, cleavable masses of rhodochrosite are highly prized by mineral collectors

for their ornamental value. Rhodochrosite is a minor or of manganese (Mn) used in the

manufacture of steel and other industrial alloys

RHODONITE:

Composition: MnSiO3; single-chain inosilicate; a pyroxenoid mineral with rotated single

chains; small amounts of magnesium (Mg+2

), iron (Fe+2

), calcium (Ca+2

) or zinc (Zn+2

) may

substitute for manganese (Mn+2

)


o, γ = 83

o

Crystal Habit: scarce individual crystals are typically tabular; more commonly occurs as

massive or granular aggregates


Hardness: 4½ – 5; moderate


Cleavage/Fracture: two orientations of pinacoidal cleavage {110} and {ī10} near right

angles (88o and 92

o); perfect


Colors: commonly rose red to pink; also reddish brown

Streak: white

Luster: vitreous; subvitreous to dull in massive aggregates

Other: commonly alters to black manganese oxides and hydroxides (wad)

Diagnostic Properties: the pink to rose red color distinguishes rhodonite from other

pyroxenoids; the near right angle cleavage and hardness distinguishes it from the softer,

rhombohedral mineral rhodochrosite and from the harder pink epidote group minerals

thulite and piemontite which possess only one orientation of cleavage




Color/Pleochroism: colorless to pale pink and yellowish red; somewhat pleochroic

Birefringence: 0.011 – 0.017; low; maximum colors are first order yellow to red

Twinning: lamellar twins may be observed

Optic Sign: Biaxial (+); 2Vz = 63 – 87o; high to very high 2V

Other: all sections display inclined extinction; some crystals are zoned

Diagnostic Properties: the combination of pale pinkish colors (if present), high

refractive indices and relief, two cleavages near right angles, low birefringence, common

twinning and biaxial (+) optics with large 2V are characteristic, as is the common

association with other manganese-rich minerals

Occurrences/Associations: Most rhodonite occurs in manganese-rich hydrothermal veins or

replacement deposits, in contact metamorphic rocks associated with such hydrothermal systems

or in metamorphosed, manganese-rich iron formations. Associated minerals include

rhodochrosite, wad, quartz, calcite, galena, sphalerite and chalcopyrite.

Uses: Rhodonite of nice color is used as an ornamental or decorative stone.

RIEBECKITE:

Composition: Na2Fe3Fe2Si8O22(OH)2; double-chain inosilicate; sodic amphibole group

mineral; riebeckite forms a complete solid solution series with glaucophane

Na2Mg3Al2Si8O22(OH)2 in which magnesium (Mg+2


) and

aluminum (Al+3

) substitutes for ferric iron (Fe+3

); riebeckite in which >50% magnesium (Mg+2

)

substitutes for ferrous iron (Fe+2

) is called magnesioriebeckite and glauconite in which >50%

ferrous iron (Fe+2

) substitutes for magnesium (Mg2) is called ferroglaucopahne; intermediate

compositions with 30 – 70% each of the reibeckite and glaucophane end members are called

crossite [Na2(Mg,Fe)3(Al,Fe)2Si8O22(OH)2]

Crystal System: Monoclinic: (2/m): a = 9.7 – 9.8Å, b = 17.9 – 18.1Å, c = 5.3Å; α = 90o, β =

104o, γ = 90

o

Crystal Habit: occurs in capillary to filiform crystals; in fibrous aggregates known as

crocidolite; also as acicular to thin prismatic to bladed crystals; as massive to granular

aggregates and as disseminated crystals;



Specific Gravity: 3.4 – 3.5; high side of moderate


(56o and 124

o); very good; splintery; not discernible in finely crystalline aggregates

Diaphaneity: barely translucent

Colors: typically dark, bluish black to black

Streak: gray to blue

Luster: vitreous and very reflective in crystals

Other: fibers in crocidolite are flexible; the chatoyant fibrous stone called “tiger-eye” is

formed when quartz replaces crocidolite

Diagnostic Properties: The bluish tint to the nearly black color and high reflectivity, in

combination with the hardness and cleavage characteristic of other amphiboles, is

characteristic



Relief: moderately high, positive

Color/Pleochroism: pleochroic; deep blue, indigo blue, blue, violet, greenish blue,

yellow, yellow-green, yellow-brown, smoky green; deep absorption when c-axis is

parallel to the vibration direction of the lower polar

Birefringence: 0.12 – 0.20; low plus; maximum colors are first order red and are

obscured by the strong colors

Twinning: simple pairs and lamellar twins common

Optic Sign: Biaxial (–); 2Vx = 62 – 85o; high to very high 2V; magnesioriebeckite may

be biaxial (+)

Other: longitudinal sections and acicular crystals are length fast

Diagnostic Properties: the diamond-shaped cross-section and two cleavages at 56o and 124

o

distinguish riebeckite as an amphibole; the deep blue colors, with strong absorption (nearly black

in some orientations) are characteristic of riebeckite and distinguish it from other blue

amphiboles such as glaucophane which has paler colors, is length slow, possesses lower

refractive indices and relief and has larger extinction angles; tourmaline is uniaxial, so possesses

parallel extinction

Occurrences/Associations: Riebeckite forms in both igneous and metamorphic rocks. It is a

fairly common mineral in alkali-rich, peralkaline igneous rocks such as foid syenite, alkali

syenite and alkali granite and pegmatite, where it is associated with albite-rich plagioclase, K-

feldspars, aegerine, aegerine-augite, arfvedsonite and quartz or feldspathoids. It also occurs in

metamorphosed banded iron formation (BIF), in many cases as fibrous crocidolite. Less

commonly it has been reported from rocks of the blueschist facies formed under high P/T

conditions in subduction zones.

Uses: Crocidolite was used as a source of asbestos for many years, but its health risks seem to be

even worse than those for chysotile (serpentine) asbestos. Tiger eye is used in jewelry.

ROMANECHITE: see WAD

RUBELLITE: see TOURMALINE

RUBY: see CORUNDUM

RUTILE:

Composition: TiO2; an oxide mineral; polymorphic with brookite and anatase; some iron (Fe+2

Fe+3

) and tin (Sn+4

) may substitute for titanium (Ti+4

), as may niobium (Nb+5

) and tantalum (Ti+5

)

Crystal System: Tetragonal (4/m2/m2/m): a1 = 4.59Ǻ, a2 = 4.59Ǻ, c = 2.96Ǻ; α = 90o, β = 90

o,

γ = 90o

Crystal Habit: small, often slender, striated tetragonal prisms with dipyramid {hhl} or {h0l}

terminations and acicular crystals; also in massive to granular aggregates and as disseminated

crystals or grains



Specific Gravity: 4.2 – 4.3; high; higher if rich in tin, niobium, tantalum or iron

Cleavage/Fracture: two orientations at 90o; prismatic {110}; good only; may show

conchoidal or uneven fracture


Colors: typically reddish brown to black; violet and yellow-green tints possible

Streak: white

Luster: submetallic to adamantine

Other: commonly alters to leucoxene, a mixture of iron and titanium oxides and

hydroxides

Diagnostic Properties: The red brown to red black color; adamantine to submetallic

luster are characteristic; the striated prismatic to acicular crystal tetragonal crystals are

clinchers; cassiterite is similar, but has a higher specific gravity and commonly occurs in

botryoidal aggregates; cuprite has a higher specific gravity, a reddish streak and is softer




Color/Pleochroism: yellow brown to reddish brown; darker with higher iron content;

weakly pleochroic

Birefringence: 0.28; extremely high; creamy white upper order interference colors are

masked by color

Twinning: cyclic contact twins common

Optic Sign: Uniaxial (+); high relief makes figures difficult to obtain

Diagnostic Properties: the extremely high refractive indices and relief, dark yellow-

brown to reddish-brown color, two decent cleavages at right angles (if visible), extreme

birefringence, slender prismatic-acicular tetragonal crystals with parallel extinction and

(if obtainable) uniaxial (+) optics are characteristic; cassiterite is quite similar to rutile,

but is often zoned or with splotchy yellow-brown-orange-red colors, has much less

extreme relief, lacks cleavage and commonly occurs in radial-fibrous or colloform

aggregates hematite is deep red, hexagonal and nearly opaque

Occurrences/Associations: Rutile is widely disseminated as an accessory mineral in a variety of

igneous and metamorphic rocks. Because it is quite resistant to weathering, it is concentrated in

the heavy mineral population of detrital sediments, sometimes as placer ore deposits associated

with monazite, ilmenite, cassiterite, and zircon. Large crystals typically occur only in coarse

granitoids and pegmatites and in high temperature veins with quartz and apatite.

Uses: Rutile is a major ore of titanium (Ti) which is used with iron to strengthen steel and in

alloys with aluminum, molybdenum and vanadium that are used in aircraft engines, missiles and


boats. Because it is physiologically inert and can integrate with bones, titanium is a choice

material for joint replacement and tooth implants. Because of its light weight and strength,

titanium is used in alloys for golf clubs and eyeglass frames. Titanium is used most extensively

as a pigmenting agent in white paints and plastics.

SANIDINE:

Composition: KAlSi3O8; a tectosilicate mineral; a feldspar; polymorphic with microcline and

orthoclase; significant sodium (Na+1


) at the time of formation,

but during cooling much sodium (Na+1

) may exsolve to form albite, so sanidine is sometimes

perthitic, microperthitic or cryptoperthitic; barium (Ba+2

) and rubidium (Rb+2

)may also substitute

for potassium (K+1

) if the substitution is balanced by aluminum (Al+3

) for silicon (Si+4

); sanidine

commonly has a low degree of Al-Si ordering, so has a more disordered crystal structure than

orthoclase or well ordered microcline


o, γ =

90o

Crystal Habit: typically as tabular to flattened prismatic crystals; commonly as disseminated

crystals (phenocrysts) in volcanic rocks; also as acicular crystals in spherulites formed by the

divitrification of glass


Hardness: 6; hard


Cleavage/Fracture: 2 cleavage orientations at right angles; one perfect; one good to very

good

Diaphaneity: nearly transparent to quite translucent

Colors: typically colorless to white

Streak: white

Luster: vitreous

Other: sometimes perthitic, with blebs, stringers and/or patches of intergrown albite, but

less often than microcline and orthoclase; commonly alters to sericite and clay minerals

Diagnostic Properties: distinguished as a potassium feldspar by its hardness,

sometimes perthitic nature and two sets of cleavage at right angles; sanidine is

generally more transparent than microcline or orthoclase; sanidine is generally colorless

to white, whereas both microcline and orthoclase are often salmon pink and microcline is

sometimes blue-green; microcline is also more commonly perthitic and crystallizes in the

triclinic system; sanidine can otherwise be distinguished from the other potassium

feldspars by optical microscopy


Refractive Indices: nα = 1.518; nβ = 1.522 – 1.523; nγ = 1.523 – 1.524


Color/Pleochroism: colorless; often quite clear


Twinning: paired Carlsbad penetration twins common; also simple, paired Mannebach

and Baveno twins; lacks polysynthetic pericline and albite twins and therefore the

gridiron (scotch plaid or tartan) twinning characteristic of microcline

Optic Sign: Biaxial (–); 2Vx = 0 - 45o; low to moderate 2V; increases with degree of

order and with sodium (Na+1

) content

Other: typically microperthitic; phenocrysts are commonly zoned

Diagnostic Properties: the combination of low negative relief, low birefringence, biaxial

(–) optics with a small 2V, common simple paired twins and occurrence in volcanic

and hypabyssal igneous rocks are characteristic; orthoclase is similar, but generally more

“cloudy”, possesses a significantly larger 2V and has a much less restricted occurrence;

microcline typically possesses polysynthetic, tapered pericline twins and albite twins in a

gridiron pattern undercrossed polars; quartz is untwinned and uniaxial (+) and rarely

forms in volcanic and hypabyssal environments

Occurrences/Associations: Sanidine is less common than orthoclase and microcline. It is

abundant as phenocrysts and groundmass material in alkali-rich volcanic rocks of rhyolite, alkali

rhyolite, trachyte, alkali trachyte, and rhyodacite composition that formed form at rapid cooling

rates at or very near the surface. Sanidine also occurs in rare, very high temperature contact

metamorphic rocks of the sanidinite facies. It is also a minor component of detrital sedimentary

rocks including arkosic and volcanoclastic sandstone.

Uses: Because sanidine is a scarcer mineral than microcline or orthoclase, it is less often used for

economic purposes. Ground up and mixed with clays and quartz, it does find use in the

manufacture of glass and ceramics products including tiles, plumbing fixtures and electrical

insulators and as filler in paints, plastics and paper products

SAPPHIRE: see CORUNDUM

SCAPOLITE:

Composition: [(Na,Ca)4(Si,Al)(AlSi2O6)3(Cl,CO3); a tectosilicate mineral; a complete solid

solution series exists between end members marialite [Na4(AlSi3O6)3Cl] and meionite

[Ca4(Al2Si2O6)3CO3]; most scapolite has substantial amounts of both end members; potassium

(K+1

) commonly substitutes for sodium (Na+1

); small amounts of sulfate (SO4-2

) may substitute

for carbonate (CO3-2

) and fluoride (F-1

) and hydroxyl ion (OH-1

) for chloride (Cl-1

)

Crystal System: Tetragonal (4/m): a1 = 12.06Å, a2 = 12.06Å, c = 7.57Å; α = 90o, β = 90

o, γ =

90o

Crystal Habit: typically as prismatic crystals with squarish 4-8 sided sections that can be quite

large; commonly in granular aggregates or as disseminated crystals




Cleavage/Fracture: 2 orientations of prismatic {110} cleavage at right angles; good

only; another set of cleavage intersecting the first two at 45o is rarely discernible but

often gives cleavage fragments a slightly fibrous or “striated” appearance


Colors: rarely colorless; typically white to greenish or grayish; less commonly yellow,

bluish or even pink

Streak: white


Other: commonly fluoresces orange to yellow under ultraviolet radiation; alters to

calcite, epidote, chlorite, sericite and zeolite minerals

Diagnostic Properties: the prismatic crystals with square cross-sections and the fibrous

appearance of the cleavage surfaces, taken with the light color and hardness are

characteristic; the extra set of cleavages which produce a fibrous appearance on the

prominent cleavages distinguish scapolite from most feldspar


Refractive Indices: nω = 1.532 – 1.607; nε = 1.522 – 1.571; increasing with meionite

content

Relief: low negative for marialite-rich samples to moderate positive for meionite-rich

samples; low positive for most scapolite


Birefringence: 0.004 – 0.038; increases with meionite content; extremely variable from

low with first order grays and white for marialite-rich samples to high with second and

third order colors for meionite-rich samples; most scapolite has low to moderate

birefringence

Twinning: none


Other: longitudinal sections are length fast and display parallel extinction

Diagnostic Properties: the lack of color, generally low relief, decent right angle

cleavages, generally low to moderate birefringence and uniaxial (–) and length fast optics

are characteristic; the tetragonal prismatic crystals are very helpful where visible; typical

scapolite resembles other colorless minerals with low relief and relatively low

birefringence; quartz lacks cleavage, is hexagonal and is uniaxial (+), so length slow;

nepheline lacks cleavage and has negative relief; cordierite is biaxial, usually (-) and

often twinned and lacks cleavage

Occurrences/Associations: Scapolite most commonly occurs in low- to medium-grade

calcareous and mafic metamorphic rocks produced by both contact and regional metamorphism.

In marble and skarn, it is commonly associated with calcite, garnet, diopside, actinolite, apatite

and titanite. In amphibolites and related rocks it is associated with hornblende, calcic

plagioclase, augite, epidote and titanite. Scapolite also occurs in high-grade metamorphic rocks

of the granulite and pyroxene hornfels facies.

Use: Scapolite has little economic value. Rare colorless crystals are sometimes used as

semiprecious gemstones.

SCHORL: see TOURMALINE

SELENITE: see GYPSUM

SCHEELITE:

Composition: CaWO4; a tungstate mineral

Crystal System: Tetragonal (4/m): a1 = 1.92Ǻ, a2 = 1.92Ǻ, c = 1.94Ǻ; α = 90o, β = 90

o, γ = 90

o

Crystal Habit: tabular to stubby prismatic tetragonal dipyramidal {101} and {112} crystals that

somewhat resemble isometric octahedra; more typically in granular to massive aggregates


Hardness: 4 ½ - 5


Cleavage/Fracture: 2 orientations at 90o; good only

Diaphaneity: somewhat translucent to transparent

Colors: typically white to pale shades of yellow, green or brown

Streak: white

Luster: vitreous (subvitreous) to adamantine

Other: fluoresces a bluish color under short wave ultraviolet light

Diagnostic Characteristics: The combination of very high specific gravity for a non-

opaque mineral and the blue fluorescence are helpful in distinguishing scheelite from

other non-opaque minerals. When present, the tetragonal dipyramidal crystals are also

indicative.



Relief: extremely high


Birefringence: 0.017; low plus; maximum first order yellow to red colors

Twinning: contact and penetration twins common

Optic Sign: uniaxial (+)

Other: longitudinal sections are length slow and exhibit parallel extinction

Diagnostic Properties: the combination of lack of color, extreme relief, cleavage,

common twinning, relatively low birefringence and uniaxial (+) optical properties are

characteristic, as is the limited occurrence and associations; zircon has similar crystal

forms and relief, but has much higher birefringence, brownish colors in hand-specimen

and lacks good cleavage

Occurrences/Associations: Scheelite occurs principally in skarn deposits produced by the

contact metamorphism of carbonate rocks in contact with silicic intrusions where it is associated

with calcite and calcsilicate minerals such as wollastonite and diopside. It is also reported from

granitic pegmatites and from high-temperature hydrothermal vein and replacement deposits

where it is associated with cassiterite, wolframite, molybdenite, topaz, apatite, fluorite and

quartz.

Uses: Scheelite is a major ore of tungsten (W) used primarily to harden steel for the manufacture

of tools used for cutting and drilling, especially power tools. Tungsten is also used in the

manufacture of silicon carbide, an abrasive harder than any mineral except diamond, which is

used wherever hard substances are required. Pure tungsten has long been used for filaments in

lamps and light bulbs.

SERPENTINE: (aka: SERPENTINE GROUP)

Composition: Mg3Si5O10((OH)4; a phyllosilicate mineral (group) with a T-O structure; some


) for silicon (Si+4

) and aluminum (Al+3

), ferric iron (Fe+3

) or

ferrous iron (Fe+2


) for magnesium (Mg+2

) is fairly common; three main

varieties of serpentine are micro-fibrous chrysotile, commonly scaly to foliated antigorite, both

monoclinic (data given below) and typically massive triclinic lizardite; the latter two cannot

always be distinguished without x-ray crystallography; all three are typically intergrown; many

other varieties are recognized; other varieties exist

Crystal System: Monoclinic (2/m): a = 5.31 – 5.32Å, b = 9.12 – 9.21Å, c = 14.45 – 14.64Å; α

= 90o, β = 93

o, γ = 90

o

Crystal Habit: chrysotile occurs in capillary to filiform crystals, typically in fibrous aggregates

that are sometimes banded; antigorite occurs in scaly crystals, typically in foliated to radial,

sheaf-like aggregates, but can occur in fibrous aggregates; lizardite typically occurs in finely-

crystalline massive aggregates


Hardness: 2 ½ - 4; low to moderate; some varieties even harder

Specific Gravity: 2.5 – 2.7; moderate; iron rich varieties may be higher

Cleavage/Fracture: one orientation of basal {001} cleavage; excellent; but generally

indistinct, discernible only in some antigorite


Colors: typically shades of green, with color variation (“mottling”) common; light to

dark green, yellow green or grayish colors are common; less commonly reddish

Streak: white

Luster: waxy; silky in fibrous varieties

Other: commonly alters to chlorite

Diagnostic Properties: yellow-green, silky, microfibrous chrysotile is distinctive; softer

than fibrous amphiboles; hardness, greenish color mottling, waxy luster and occurrence

in altered ultramafic rocks is useful in recognizing lizardite/antigorite


Refractive Indices: nα = 1.529 – 1.595; nβ = 1.530 – 1.603; nγ = 1.537 – 1.604; higher

values (nα > 1.557; nβ > 1.563; nγ = 1.564) are generally associated with antigorite and

lizardite, lower values with chrysotile

Relief: low negative to low-moderate positive

Color/Pleochroism: colorless to pale shades of green and yellow-green; very weakly

pleochroic in some instances

Birefringence: 0.002 – 0.012; low; maximum colors in first order gray to yellow (may

look nearly isotropic) for most orientations; anomalous greenish-yellow colors in occur in

some samples, especially of antigorite

Optic Sign: Biaxial (–); 2Vx = 20 – 50o; low to moderate; rarely biaxial (+) in some

chrysotile; figures often difficult to obtain because of the finely crystalline nature of most

serpentine

Other: commonly pseudomorphic after olivine and pyroxenes; fibers in chrysotile are

length slow; antigorite typically displays mottled or wavy, undulatory extinction;

the blades in antigorite are length slow

Diagnostic Properties: the pale green-yellow green color, generally low relief and low

birefringence and capillary to scaly crystal habit are characteristic; most other fibrous,

asbestiform minerals, including tremolite, actinolite, anthophyllite and crocidolite

(riebeckite) possess significantly higher relief and higher birefringence; brucite possesses

has higher indices, slightly higher birefringence, deep brown anomalous colors, whorl

structure, length slow scales and is uniaxial (+); chlorite is similar to antigorite, but

displays cleavage more often, often displays blue-violet abnormal interference colors, is

mostly biaxial (+) and has somewhat higher relief; talc possesses much higher

birefringence; clear distinction between serpentine varieties requires other analytical

techniques such as XRD; serpentine is similar in many respects, but tends to have

different crystal habits (fibrous or sheaf-like), displays cleavage less often and is rarely

biaxial (+)

Occurrences/Associations:

Uses: Chrysotile was once extensively mined as a source of asbestos and still is to a lesser

extent. Its fibrous habit, flexibility, low thermal conductivity and resistance to combustion made

it valuable in the manufacture of materials used for insulation and fireproofing. It was widely

used in insulating material, in the manufacture of brake shoes and clutch linings, as an insulator

for hot water pipes and boilers, as a reinforcing material in floor, ceiling and roof tiles and even

as a significant material in felt hats. However, its micro-fibrous nature makes it an extremely

dangerous carcinogen when inhaled into the lungs and can cause other forms of severe lung

damage. Because of this, it is less widely used and mined than it was several decades ago.

Lizardite is widely used as an ornamental stone in carvings (e.g., by northern Native Americans)

and, when it occurs naturally mixed with marble to form verde antique, as a decorative building

stone.

SIDERITE:

Composition: FeCO3; a carbonate; isostructural with calcite, magnesite, rhodochrosite and

smithsonite, all members of the rhombohedral (calcite) group of carbonate minerals; complete

solid solution with rhodochrosite and partial solid solution with the others

Crystal System: Hexagonal (Rhombohedral): (bar32/m): a1 = 4.69Ǻ, a2 = 4.69Ǻ, c = 15.38Ǻ;

α = 120o, β = 90

o, γ = 120

o

Crystal Habit: equant rhombohedral crystals {10bar11}; coarsely crystalline massive and

granular aggregates common; also occurs as ooids and in concretions; less commonly as acicular

to capillary crystals in radiating, colloform (botryoidal) aggregates


Hardness: 4 – 4 ½

Specific Gravity: 3.96; moderately high; lower where manganese (Mn) and/or

magnesium (Mg) substitute substantially for iron (Fe)

Cleavage/Fracture: 3 orientations, not at 90o (73

o and 107


Diaphaneity: translucent to quite translucent

Colors: light to medium-dark brown; sometimes with yellowish or reddish tint

Streak: white


Other: oxidizes to iron oxyhydroxides (goethite, limonite) and oxides (hematite);

effervesces in HCl only if powdered or acid heated

Diagnostic characteristics: the brown color and elevated specific gravity distinguish

siderite from other carbonates with rhombohedral cleavage



Relief: variable; from moderate to very high positive; sections in which both nω and nε

are visible produce a “twinkling” effect as the relief changes when stage is rotated under

plane light

Color/Pleochroism: colorless to pale yellow brown (look closely!); slightly pleochroic


orientations

Twinning: rhombohedral twins fairly common





uniaxial (–) optics are characteristic of rhombohedral carbonates such as siderite; siderite

is distinguished from other rhombohedral carbonates, except rhodochrosite and

smithsonite, by its very high maximum refractive indices and relief and its consistently

positive relief in all orientations; siderite is often yellow-brown (from oxidation of iron)

under plane light, whereas rhodochrosite is colorless to pale pink; siderite displays yellow

brown to brown colors in hand specimen, rather than the pink-rose red colors shown by

rhodochrosite; smithsonite is colorless and typically occurs in colloform-globular

aggregates

Occurrences/Associations: Siderite is fairly common in sedimentary iron deposits where it

occurs as ooids and/or concretions, as layers and lenses in mudrocks, in metamorphosed

Precambrian banded iron formations (BIF) and as hydrothermal replacement deposits in

limestones. Siderite also occurs in hydrothermal veins where it is associated with galena, pyrite,

tetrahedrite, chalcopyrite and bornite.

Uses: Siderite is a minor, but not insignificant, ore of iron used in the manufacture of steel and

related products.

SILLIMANITE:

Composition: AlAlOSiO4; a nesosilicate (orthosilicate) mineral, polymorphic with andalusite

and kyanite; small amounts of iron (Fe+3

) may substitute for aluminum (Al+3

)


90o, γ = 90

o

Crystal Habit: typically in slender prismatic to acicular crystals with diamond-shaped cross-

sections; commonly in fibrous to “swirled”, interlaced aggregates called fibrolite; also in

foliated aggregates and as disseminated crystals


Hardness: 6½ - 7½; hard to very hard


Cleavage/Fracture: one orientation of pinacoidal {100} cleavage; perfect; but may be

very difficult to discern in fibrous aggregates


Colors: most commonly white to creamy white; also colorless or pale yellow brown

Streak: white

Luster: pearly or silky to vitreous

Other: alters to sericite and chlorite

Diagnostic Properties: The white color, hardness, slender tabular to acicular crystals in

fibrous to swirled masses are characteristic; tremolite and wollastonite can have a similar

appearance, but are associated with calcsilicate and carbonate minerals, rather than with

the pelitic suite of cordierite, garnet, corundum and biotite





Birefringence: 0.018 – 0.022; moderate; maximum lower second order blues and greens,

but often first order yellow or red

Twinning: none reported

Optic Sign: Biaxial (+), 2Vz = 20 - 30o; low 2V

Other: basal sections of prismatic crystals are diamond-shaped, possess symmetrical

extinction and display the cleavage parallel to the long diagonal; fibers and longitudinal

sections of slender prisms are length slow and display parallel extinction

Diagnostic Properties: the combination of lack of color, high relief, moderate

birefringence, fibrous to “swirled” habit, single cleavage parallel to the diagonal of the

basal section, length slow character and biaxial (+) optics with low 2V are diagnostic;

kyanite possesses angular extinction, higher relief, is commonly twinned and is typically

bladed to prismatic; andalusite is typically pleochroic in pale yellow-green-pink shades,

has two sets of decent cleavage near right angles, is length fast, is biaxial (–) with a large

2V and in the variety “chiastolite” displays symmetrically arranged carbonaceous

graphite inclusions

Occurrences/Associations: Sillimanite is the high temperature polymorph of aluminum silicate

and typically forms in pelitic metamorphic rocks at temperatures below 550oC over a large range

of pressures. Sillimanite is abundant in regional metamorphic rocks such as schist, gneiss and

granulite produced along Buchan and Barrovian trajectories in the granulite facies where it is

associated with cordierite, garnet, corundum, and biotite. It is also common in rocks pelitic rock

formed by contact metamorphism in the pyroxene hornfels facies. Sillimanite also occurs in

peraluminous igneous rocks including granitoids and pegmatite, although whether or not it is a

primary mineral in such rocks is controversial.

Uses: Sillimanite is mined less commonly than kyanite and andalusite to be refined for use as a

refractory material used in the manufacture of the porcelain used spark plugs and high alumina

bricks for lining blast furnaces and kilns. It may also be used as an abrasive.

SILVER:

Composition: Ag; a native element (native metal); forms a complete solid solution series with

gold (Au); samples with high gold content are called electrum.


o, γ

= 90o

Crystal Habit: filiform or equant, the latter poorly formed octahedral {111} dodecahedral {011}

and cubic {001} crystals; arborescent or dendritic aggregates; also as disseminated flakes and

irregular masses.


Hardness: 2½-3 (soft)



Diaphaneity: opaque

Colors: silvery white; tarnishes dark gray to black

Streak: silvery white

Luster: metallic

Other: sectile, malleable and ductile

Diagnostic Properties: The silvery white color, low hardness, sectility and malleability,

and tarnish distinguish silver from most silver gray metallic minerals such as stibnite,

enargite, pyrargerite, argentite and arsenopyrite.

Microscopic Properties: Opaque; bright, creamy white in reflected light; black tarnish

Occurrences/Associations: Many silver deposits occur in the oxidized zone of low temperature

hydrothermal sulfide veins associated with intermediate to acidic igneous rocks where it forms

by the oxidation of silver-bearing sulfide, sulfarsenide and sulfantimonide minerals such as

tennantite, tetrahedrite, acanthite, argentite, pyrargerite and proustite. Native silver also occurs as

a primary mineral in low-medium temperature hydrothermal veins where it is associated with

pyrite, galena, sphalerite, chalcopyrite, cobaltite, nickelite, tetrahedrite and silver sulfides.

Use: An important ore of silver used in jewelry and in electrical components in computers.

Silver has achieved significant value as a monetary hedge against inflation and political unrest.

Silver was formerly widely used in coinage and in the manufacture of photographic film; both of

these uses have been considerably curtailed.

SMECTITE:

Composition: Ca0.17(Al,Fe,Mg)2(Si,Al)4(OH)2•nH2O; a phyllosilicate mineral; a 10Å to 15Å

clay mineral with an expandable T-O-T structure that can accommodate large interlayer

molecules including calcium (Ca+2

), but also water (H2O), which causes the layer dimensions to

swell; when dehydrated, the dimensions shrink back to ~10Å; sodium (Na+1

) may substitute for

calcium (Ca+2

) in addition to the substitutions indicated by the formula; small variations in

smectite composition and structure yield clay minerals that include montmorillonite,

nontronite, hectorite and saponite.

Crystal System: Monoclinic: (2/m): a = 5.18Å, b = 9.08Å, c = 10 – 15.2Å ; α = 90o, β = 90+

o, γ

= 90o


crystalline, massive aggregates or as disseminated crystals or grains


Hardness: 1 – 2; soft to very soft

Specific Gravity: ~2.0 – 3.0, depending on water content; low to moderate

Cleavage/Fracture: one orientation of basal pinacoidal cleavage; usually indistinct

Diaphaneity: typically somewhat to barely translucent

Colors: typically gray to green to whitish-gray; may be stained yellow-brown to reddish

brown by limonite

Streak: white to light gray

Luster: dull, earthy

Other: very “sticky”, especially when wet; often displays desiccation cracks when dry

Diagnostic Properties: Smectite is distinguished from other clay minerals with which it

is commonly mixed only by advanced methods such as XRD



with iron content

Relief: low negative to moderate positive

Color/Pleochroism: colorless to pale yellow-green

Birefringence: 0.010 – 0.038; low to moderately high; maximum colors from first order

yellow to lower third order

Optic Sign: Biaxial (–); 2Vx = 0 -10o; small 2V; figures rarely obtained due to small

crystal size

Diagnostic Properties: the microscopic, platy crystals, sometimes pale yellow-green

color, low-moderate relief and moderate birefringence distinguish smectite; kaolinite

possesses lower birefringence and a larger 2V (if a figure can be obtained); illite clays

are difficult to distinguish from colorless smectite; there is a large variety of clay

minerals and most cannot be distinguished optically and require alternate techniques for

accurate identification

Occurrences/Associations: Smectite clays form chiefly from the alteration of aluminum-

bearing, ferromagnesian silicate minerals especially hornblende, but also biotite and calcium-

bearing clinopyroxene and several metamorphic minerals. They are produced either by

hydrothermal alteration of such minerals or by weathering. They are therefore an abundant

constituent of soils and of detrital sedimentary rocks, especially shale and mudstone. Another

important occurrence of smectite clay (“montmorillonite”) is in bentonite which forms by the

alteration of volcanic ash strata in marine environments.

Uses: Smectite is an important constituent of drilling mud in which it is used to increase the

viscosity. Because it swells when wetted, smectite has long been used as a short-term patch to

stop leakage in rock walls and dams. It is also used as an absorbent to remove various chemicals

from water supplies. Because of their water-absorbing and releasing capabilities, smectite clays

can rapidly change volume, leading to severe subsidence of surface structures and significant

slope stability problems, as discussed in chapter 12 of the textbook.

SMITHSONITE:

Composition: ZnCO3; a carbonate; isostructural with calcite, magnesite, rhodochrosite and

siderite, all members of the rhombohedral (calcite) group of carbonate minerals; partial solid

solution with all of these


= 120, β = 90 o, γ = 120

o

Crystal Habit: equant rhombohedral {10bar11} to prismatic scalenohedral {11bar20} crystals;

very commonly in colloform (reniform-globular) aggregates; also as drusy encrustations or

granular or earthy masses


Hardness: 4 – 4½

Specific Gravity: 4.3-4.4; high

Cleavage/Fracture: 3 orientations at 90o

(72o and 108


generally not visible in aggregates


Colors: variable (allochromatic); may be colorless to white; but commonly green; also

pink, blue or yellow

Streak: white

Luster: vitreous

Other: effervesces readily in dilute HCl

Diagnostic characteristics: Its ready effervescence in HCl and common reniform-

globular habit distinguish it from most rhombic carbonates and its elevated specific

gravity and higher hardness distinguish it from calcite



Relief: variable; from moderate to very high positive; sections in which both nω and nε are

visible produce a somewhat “twinkling” effect as the relief changes when stage is rotated

under plane light



orientations

Twinning: lacks mechanical twinning



“twinkling effect”, the extreme birefringence, the rhombohedral cleavage (if visible), and

the uniaxial (–) optics are characteristic of rhombohedral carbonates such as

smithsonite; much smithsonite occurs in colloform (reniform-globular) aggregates; other

rhombohedral carbonates generally do not; only siderite possesses similarly high indices

Occurrences/Associations: Most smithsonite forms in the oxidized zone of hydrothermal vein

deposits where it forms by the oxidation of sphalerite (ZnS) and is commonly associated with

cerussite, hemimorphite, calcite and limonite.

Uses: A minor ore of zinc (Zn) used in the manufacture of galvanized steel (alloyed with iron),

brass (alloyed with copper) and sheet zinc. Zinc is also used in conventional batteries and as an

additive in many paints.

SPECULARITE: see HEMATITE

SPESSARTINE (SPESSARTITE): see GARNET

SPHENE: old name for TITANITE

SODALITE: (SODALITE GROUP)

Composition: Na8(AlSiO4)6Cl2; a tectosilicate mineral; a feldspathoid mineral; some

substitution of potassium (K+1

) for sodium (Na+1

); closely related, scarcer sodalite group

minerals include hauyne {(Na,Ca)4-8[AlSi(O,S)4]6(SO4)1-2} and [nosean Na8(AlSiO4)6(SO4)].

Lazurite, a significant constituent of the valuable decorative stone lapis lazuli is a blue variety

of hauyne in which chlorine (Cl-1

) and sulfur (S-2

) substitute for sulfate ion (SO4)-2

.

Crystal System: Isometric (bar43m) a1 = 8.87, a2 = 8.87Å, a3 = 8.87Å; α = 90o, β = 90

o, γ =

90o

Crystal Habit: rare individual crystals are equant dodecahedra (110); more commonly in

massive to granular aggregates or as disseminated crystals



Specific Gravity: 2.3; Low

Cleavage/Fracture: not discernible; irregular to conchoidal fracture


Colors: typically light to dark blue, but also gray; also white, pale yellow or pale green

Streak: white; lazurite has a bluish streak

Luster: typically vitreous; somewhat greasy in gray varieties

Other: commonly alters to cancranite (often yellow), calcite, clay minerals and zeolites

Diagnostic Properties: the blue color, combined with the hardness is distinctive; the

varieties of blue-colored sodalite group minerals cannot easily be distinguished

macroscopically


Refractive Indices: n = 1.483 -1.487; for hauyne (including lazurite): 1.470 – 1.495; for

nosean: 1.494 – 1.510


Color/Pleochroism: colorless to pale blue; lazurite may be deeper blue

Birefringence: not applicable; isometric

Twinning: occurs, but rarely discernable

Optic Sign: not applicable; isometric

Diagnostic Properties: the combination of moderate negative relief, diamond-shaped

cross-sections (of dodecahedra), lack of good cleavage and isotropic optics are

characteristic; sodalite sensu stricto is colorless, the bluish tones of hauyne (and lazurite)

are clinchers and the association with other feldspathoid minerals helpful; analcime is

similar, but typically occurs in trapezohedral crystals with 8-sided cross-sections and has

a different occurrence and associations

Occurrences/Associations: Sodalite and other sodalite group minerals occur chiefly in alkali-

rich, silica undersaturated plutonic and volcanic igneous rocks including foid syenite, phonolite,

alkali basalt and foid basalt. Sodalite and lazurite also occur in some skarns produced by contact

metamorphism of impure carbonate rocks.

Uses: The principle use of sodalite (and lapis lazuli) is as an expensive dimension stone used to

face structures and for other high-end decorative purposes.

SPHALERITE:

Composition: ZnS; a sulfide, significant amounts of Fe may substitute for Zn; smaller amounts

of cadmium and rarer metals such as gallium, germanium and indium also substitute

Crystal System: Isometric (bar43m); a = 5.41Ǻ, b = 5.41Ǻ, c = 5.41Ǻ; α = 90o, β = 90

o, γ =

90o

Crystal Habit: occurs in equant tetrahedral crystals; cubic and dodecahedral crystals also occur

in malformed crystals; common as coarsely crystalline, cleavable masses and granular aggregates

and as disseminated crystals


Hardness: 3½ - 4; intermediate

Specific Gravity: 3.9 – 4.1; moderately-high; increases with iron content

Cleavage/Fracture: 6 orientations not at 90o; dodecahedral {011}; perfect, but it is

generally very difficult to discern all six orientations on a given specimen

Diaphaneity: somewhat to barely translucent

Colors: typically yellow-brown and black in “splotchy” pattern; can be shades of red,

even green

Streak: pale yellow to brown; darker with higher iron content

Luster: vitreous to adamantine and resinous; almost submetallic in some cases

Other: gives off sulfurous odor when streaked


Refractive Indices: n = 2.37 – 2.50; increases with iron content


Color/Pleochroism: pale yellow or brown; not pleochroic because isotropic

Birefringence: isotropic; thus none

Twinning: twins may be simple pairs or complex

Optic Sign: isotropic; thus none

Diagnostic Properties: the combination of pale yellow to brown color, extreme relief,

multiple sets of cleavage intersecting in hexagonal patterns, isotropic nature and

commonly triangular, square or even diamond shaped cross-sections are characteristic;

cassiterite can have similar colors and diamond-shaped cross-sections, but lacks cleavage,

is anisotropic and has different crystal forms

Occurrences/Associations: Sphalerite occurs chiefly in hydrothermal vein and replacement

deposits generated over a wide range of temperatures from hypothermal to telethermal

conditions. Commonly associated minerals include galena, pyrite, marcasite, chalcopyrite and

many other sulfides as well as calcite, quartz, fluorite and barite.

Uses: Sphalerite is the major ore of zinc used in the manufacture of galvanized steel (alloyed

with iron), brass (alloyed with copper) and sheet zinc. Zinc is also used in conventional

batteries, as a soldering flux and as an additive in many paints. Sphalerite is also the chief source

of cadmium used in batteries and of gallium, germanium and indium used in the manufacture of

semiconductors and computer microprocessors.

SPINEL: (see also Chromite and Magnetite)

Composition: MgAl2O4; an oxide; a spinel group (XY2O4 group) mineral; forms a complete

solid solution series with hercynite (FeAl2O4); intermediate compositions are sometimes called

pleonaste; zinc (Zn+2


) may also substitute extensively for ferrous iron

(Fe+2

) and ferric iron (Fe+3

) and chromium (Cr+3


)


o,

γ = 90o

Crystal Habit: occurs as small, equant octahedral {111} crystals; sometimes modified by cubic

{001} or dodecahedral {110} faces; also as massive to granular crystal aggregates or




Specific Gravity: 3.5 – 3.9; high; increasing with higher iron content

Cleavage/Fracture: none; may display octahedral parting


Colors: variable; typically blue to green shades; also colorless, red; darker with

increasing iron

Streak: white

Luster: vitreous

Diagnostic Properties: The transparent-translucent, octahedral crystals and high

hardness are distinctive and distinguish spinel from chromite and magnetite; iron-rich

hercynite samples may resemble magnetite, but have a lighter-colored streak and are not

magnetic


Refractive Indices: n = 1.714 – 1.775; higher with increasing iron; Mg-rich spinels are

typically < 1.74, pleonaste is 1.75 – l.79 and hercynite is 1.78 -1.80

Relief: high, positive, increasing with iron content

Color/Pleochroism: variable; colorless in Mg-rich varieties, usually blue or green,

in ferrous iron-rich pleonaste and hercynite; red in manganese-rich varieties; olive

brownish in chromium-rich varieties


Twinning: simple or multiple twins occur


Diagnostic Properties: the combination of high relief, square cross sections, lack of

cleavage and isotropic nature are characteristic; the color of colored varieties is

helpful; garnet typically possesses 6-8 sided cross-sections through dodecahedral and/or

trapezohedral crystals and a “rounder” outline; knowing the occurrence and associations

can help, e.g., in ultramafic rocks the Mg-rich garnet, pyrope is usually pale pink,

whereas the Mg-rich spinel is colorless or, if Cr-rich, olive brown; a combination of

refractive indices and color helps to sort out spinel varieties, but chemical analytical

techniques provide a more accurate analysis

Occurrences/Associations: Spinel is a common accessory mineral in medium- to high-grade

pelitic and calcareous metamorphic rocks where it is associated with corundum, andalusite,

kyanite, sillimanite, phlogopite, cordierite and garnet. It is also abundant in ultramafic rocks

such as peridotite, especially those that originate in the upper mantle where it is associated with

olivine, pyroxene and phlogopite. Spinel occurs sparsely in some peraluminous pegmatites and

high temperature hydrothermal veins. Nice gem quality spinel is obtained from detrital

sediments derived from the source rocks noted above.

Uses: Nicely colored samples of spinel are used as gemstones, especially the red “ruby spinel”.

SPODUMENE:

Composition: LiAlSi2O6; single-chain inosilicate; a clinopyroxene; minor substitution of

sodium (Na+1


) for lithium (Li+1

) and of iron (Fe+3


)

commonly occurs


o, γ = 90

o

Crystal Habit: individual crystals are commonly striated, prismatic to bladed, typically with

eight-sided cross-sections; they can be very large; also occur as coarse, granular masses;

sometimes acicular




Cleavage/Fracture: two orientations of prismatic {210} cleavage near 90o (87

o and 93

o);

may have a prominent parting that bisects the cleavages

Diaphaneity: nearly transparent to translucent

Colors: typically white to greenish or grayish white; also shades of pale lilac, pink, blue,

green or yellow

Streak: white


Other: fluoresces pink, orange and yellow under both short- and long-wave ultraviolet

light

Diagnostic Properties: distinguished from other pegmatite minerals by its hardness,

light colors, near right angle cleavage and striated prismatic crystals; resembles tremolite,

especially if the parting is prominent, but tremolite occurs mostly in calcareous

metamorphic rocks, whereas spodumene occurs mostly in pegmatite



Relief: moderate-high; positive


Birefringence: 0.014 - 0.027; low to moderate; maximum colors are first order yellow-

red to second order colors

Twinning: simple, paired twins common

Optic Sign: Biaxial (+); 2Vz = 58 – 68o; moderately high 2V

Other: basal cross-sections show symmetrical extinction with respect to cleavages;

maximum extinction angle in longitudinal sections is 23o – 27

o;


cleavage near right angles distinguish spodumene from amphiboles; diopside and augite

can be similar, but possess somewhat higher birefringence and larger maximum

extinction angles and have entirely different occurrences and associations; jadeite and

omphacite are similar, but possess larger extinction angles and common only in high-

pressure metamorphic rocks

Occurrences/Associations: Spodumene is a relatively scarce mineral, common only in lithium-

rich pegmatites where it is associated with tourmaline, lepidolite (Li-rich mica), amblygonite (Li-

F phosphate), beryl, quartz, K-feldspar and albite-rich plagioclase.

Uses: Spodumene is a minor source of lithium which is used in a many types of batteries

including lithium batteries, rechargeable lithium ion batteries and experimental nannowire

batteries. It is also used as a critical agent in the thickening of oils for use in high-temperature

lubricating oils and greases. Lithium is used in the manufacture of glazes with low coefficients

of thermal expansion for ceramic ovenware. It is also utilized in various forms in the

manufacture of hygroscopic desiccants, in the treatment of patients with bipolar disorder and

depression, and as a fuel additive for fuels that propel rockets and torpedoes.

STAUROLITE:

Composition: Fe2Al9O6(SiO4)4(OH)2; a nesosilicate (orthosilicate) mineral; some magnesium

(Mg+2

) commonly substitutes for ferrous iron (Fe+2

) and some ferric iron (Fe+3

) may substitute

for aluminum (Al+3

); other substitutions are minor

Crystal System: Monoclinic (2/m): a = 7.86 – 7.87Å, b = 16.53 – 16.61Å, c = 5.63 – 5.66Å; α

= 90o, β = 90 – 90.4

o, γ = 90

o; so nearly orthorhombic (pseudo-orthorhombic)

Crystal Habit: typically in flattened prismatic to bladed pseudo-orthorhombic crystals with

prism and side pinacoid yielding six-sided, diamond shape cross-sections; penetration twins

commonly yield iron cross forms at 60o or 90

o to each other; usually as disseminated crystals and

grains or in divergent aggregates; also in massive to granular aggregates




Cleavage/Fracture: typically not discernable


Colors: typically brown; yellow-brown, red-brown, black-brown


Luster: vitreous

Other: penetration twins commonly yield iron cross forms at 60o twinned on {231} or

90o twinned on {031} to each other

Diagnostic Properties: The brown bladed pseudo-orthorhombic crystals, often with

penetration twins and elevated hardness are characteristic



as iron content increases


Color/Pleochroism: colorless to pale yellow; yellow, orange yellow, honey yellow

brown to reddish brown in iron-rich varieties; pleochroic in iron-rich varieties

Birefringence: 0.009 – 0.015; low; maximum first order yellow to red and often

somewhat obscured by the mineral’s color

Twinning: common; but only occasional observed in thin section where twin plane cut

Optic Sign: Biaxial (+); 2Vz = 80 - 90o; very high; rare (–) examples are reported

Other: basal sections are 6-sided, diamond-shaped one beveled edge; crystals are very

commonly poikiloblastic, with many inclusions of other minerals creating a sieve-like

appearance; longitudinal sections are length slow and exhibit parallel extinction

Diagnostic Properties: the combination of high relief, yellow-brown color, lack of

cleavage, poikiloblastic length slow crystals, parallel extinction, low birefringence

and biaxial (+) optics with very high 2V are characteristic; the brown tourmaline dravite

resembles staurolite, but is uniaxial (–) and length-fast, has higher birefringence and

possesses roughly triangular basal sections

Occurrences/Associations: Staurolite is a significant mineral in medium-grade, pelitic

metamorphic rocks such as schist, gneiss and hornfels. It forms by contact metamorphism under

hornblende hornfels facies conditions and more extensively by regional metamorphism in the

amphibolite facies. In these rocks, staurolite is associated with almandine garnet, biotite,

muscovite, quartz, cordierite, kyanite and chloritoid. Staurolite also occurs in the heavy mineral

fraction of detrital sedimentary rocks.

Uses: Staurolite has minor use as an abrasive in sandpaper. The crossed twins are used as

pendants by some religious groups.

STISHOVITE: see QUARTZ (Silica Group)

STIBNITE:

Composition: Sb2S3; a sulfide mineral


90o, γ = 90

o

Crystal Habit: slender prismatic, even acicular, to bladed crystals are common with striations

parallel to long axis; typically in divergent to radiating aggregates; also in massive to granular

aggregates;


Hardness: 2; soft



Diaphaneity: opaque

Colors: lead gray

Streak: gray black

Luster: metallic, even splendant

Diagnostic Properties: The prismatic-bladed crystals, single cleavage and divergent-

radiating habit distinguish most stibnite from galena which is also lead gray and soft;

Microscopic Properties: opaque; pleochroic white to grayish white in reflected light

Occurrences/Associations: Stibnite is most commonly formed in low temperature hot spring,

hydrothermal vein and replacement deposits, where it is associated with galena, sphalerite,

cinnabar, realgar, orpiment and barite.

Uses: Stibnite is the principal ore of antimony (Sb) used primarily as a pigment and in the

manufacture of glass.

STILBITE:

Composition: NaCa2Al5Si13O36•14H2O; a tectosilicate mineral; a zeolite mineral; substitution

of potassium (K+1

) for sodium (Na+1

) is common


o, γ = 90

o;

Crystal Habit: occurs as tabular to bladed crystals; typically in radiating, sheaf-like aggregates

of such crystals or as more finely-crystalline massive aggregates or disseminated crystals



Specific Gravity: 2.1 - 2.2; low



Colors: typically white; also shades of gray, yellow, brown, orange or red

Streak: white

Luster: vitreous to pearly on the cleavage surfaces

Other: cruciform twins common

Diagnostic Properties: the tabular crystals in sheaf-like aggregates, hardness and single

set of perfect cleavage with pearly luster are characteristic; finely crystalline aggregates

require XRD analysis for proper identification





Birefringence: 0.06 -0.14; low; maximum colors first order grays to yellow-orange

Optic Sign: Biaxial (–); 2Vx = 30 – 50o; moderate 2V


zeolite minerals; natrolite is similar, but is biaxial (+) and typically possesses a radial-

fibrous habit rather than a sheaf-like one; heulandite is biaxial (+), typically lacks the

sheaf-like habit; chabazite is pseudo-rhombohedral, with 3 sets of pseudocubic cleavage;

laumontite has three sets of cleavage; analcime is isotropic

Occurrences/Associations: Stilbite is a relatively widespread zeolite mineral. Coarse crystals

are especially common in cavities, including vesicles, in mafic/basic rocks such as basalt and,

less commonly, intermediate volcanic rocks such as andesite. Finely crystalline stilbite occurs

less commonly in altered felsic volcanic rocks. Stilbite is also reported from alkaline soils

formed in desert environments and in some contact metamorphic aureoles.

Uses: Stilbite, like most natural and synthetic zeolites, is used, after heating and dehydration, to

produce materials that remove water vapor from carbon dioxide, refrigerants such as Freon and

from many organic chemicals used in industrial applications. It is also used as a molecular sieve

used to separate molecules of different sizes, as in separating nitrogen from air to produce nearly

pure oxygen gas. Because zeolites can selectively absorb selected molecules, they are widely

used in applications that include reducing the hardness of water for cleaner laundry by the




STILPNOMELANE:

Composition: K0.6(Fe,FeMg)6Si8Al(O,OH)27•2-4H2O; a phyllosilicate mineral with a T-O-T

structure; substantial ferric iron (Fe+3


); charge neutrality is

accomplished by the necessary substitution of oxygen (O-2

) for hydroxyl ion (OH-1

)

Crystal System: Triclinic (ī): a = 21.8 – 22.1Å, b = 21.8 – 22.1Å, c = 17.6 – 17.7Å; α = 125o, β

= 96o, γ = 120

o

Crystal Habit: crystals are platy to tabular; occur in foliated to sheaf-like, radiating crystal

aggregates


Hardness: 3 – 4; moderate


Cleavage/Fracture: one orientation of basal pinacoidal {001} cleavage; excellent, not

perfect; possesses a second cleavage at right angles, but it is usually not discernible

Diaphaneity: quite translucent to barely translucent

Colors: typically golden to reddish brown; iron-rich varieties may be dark green to black

Streak: white to tan


Other: alters to chlorite and/or iron oxides; also to clay minerals

Diagnostic Properties: Stilpnomelane resembles biotite, but the cleavage is less perfect,

it is a bit harder than biotite, occurs more frequently in radiating sheaf-like aggregates

and crystals never display 6-sided sections


Refractive Indices: nα = 1.543 – 1634; nβ = 1.576 – 1.745; nγ = 1.576 – 1.745

Relief: low to high, positive

Color/Pleochroism: strongly pleochroic; pale brown, deep brown, reddish brown,

nearly black, greenish brown, pale yellow, golden yellow, greenish yellow, yellow

brown

Birefringence: 0.030 – 0.110; moderate to very high

Twinning: none

Optic Sign: Biaxial (–) with 2Vx ~0-40o; increasing with iron content; figures may

appear Uniaxial (–) in iron-poor examples

Other: cross-sections are length slow with nearly parallel extinction, but not birdseye

extinction

Diagnostic Properties: the combination one excellent cleavage, pleochroism in shades

of yellow, brown and green (to nearly black in iron-rich examples), nearly parallel

extinction with respect to cleavage is indicative of stilpnomelane; it is best

distinguished from biotite by lack of “birdseye” extinction, very small extinction angle

and perhaps less perfect basal cleavage; iron-rich stilpnomelane also possesses higher

birefringence, deeper colors and a larger 2V than biotite

Occurrences/Associations: Stilpnomelane commonly occurs in low-grade pelitic and less

commonly mafic, regional metamorphic rocks of the greenschist facies produced by regional

metamorphism along Buchan or Barrovian trajectories, where it is associated with muscovite,

chlorite, albite-rich plagioclase and garnet. It is also reported from high pressure, low- to

moderate-temperature (high P/T) blueschist facies metamorphic rocks produced along

Sanbagawan and Franciscan trajectories in subduction zones, where it is associated with

lawsonite, glaucophane, aragonite and pumpellyite. A third occurrence is as a constituent of

metamorphosed banded iron formations (BIF).

Uses: Stilpnomelane has no significant use or application.

STRONTIANITE:

Composition: SrCO3; a carbonate mineral; isostructural with aragonite, witherite and cerrusite,

all members of the orthorhombic (aragonite) group of carbonate minerals; rather limited solid

solution with witherite in which barium (Ba+2

) substitutes for (Sr+2

) occurs and some calcium

(Ca+2

) also substitutes for strontium


90o, γ = 90

o

Crystal Habit: typically acicular crystals in radiating to fibrous aggregates; also as columnar

crystals produced by penetration twins that produce pseudohexagonal crystals; also in massive to

granular aggregates



Specific Gravity: 3.7; moderately high

Cleavage/Fracture: two cleavages not 90o; prismatic {110} good; others poor so all

cleavages are difficult to discern; subconchoidal fracture


Colors: colorless to white; also pale green, pale yellow or pale brown

Streak: white

Luster: vitreous to silky or pearly in aggreagates


celestite

Diagnostic Characteristics: strontianite can be distinguished from aragonite by its

higher specific gravity; it most closely resembles witherite (which has a higher specific

gravity) from which it can, in large crystals, be distinguished by its two orientations of

cleavage and more clearly by a test for barium or strontium


Refractive Indices: nω = 1.521; nβ = 1.674; nγ = 1.679; but quite variable about these

averages

Relief: low-moderate negative to moderate-high positive; sections in which the alpha

vibration direction is visible produce a “twinkling” effect as the stage is rotated under

plane light


Birefringence: 0.157; variable, but very high-extreme; high order faded colors (“creamy

white”)


Optic Sign: Biaxial (–); 2Vx = 7 – 10o (very small 2V)

Other: longitudinal sections are length slow with parallel extinction



uniaxial (–); aragonite has a higher 2V, higher refractive indices and relatively poor

cleavage than witherite; witherite also possesses poor cleavage and is a much less

common mineral, with slightly higher refractive indices, relief and 2V, and possesses

a markedly higher specific gravity; cerussite is distinguished by its very high refractive

indices and relief and the lack of “twinkling” as the stage is rotated under plane light

Occurrences/Associations: Most strontianite forms in low-temperature hydrothermal veins and

replacement products in limestone where it is associated with barite, celestite and calcite. It has

also been reported from low-temperature sulfide veins that cross-cut igneous rocks.

Uses: Stontianite is minor ore of strontium, used to plate the screens of cathode ray tubes to

prevent x-ray leakage and therefore in television and computer screens. Other uses include the

production of ceramics and fluorescent lights and of the red colors in flares and fireworks

SULFUR:

Composition: S; a native element (non-metallic); small amounts of selenium (Se) may substitute

for sulfur

Crystal System: Orthorhombic (2/m2/m2/m): a = 10.47Ǻ, b = 12.87Ǻ, c = 24.49Ǻ; α = 90o, β

= 90o, γ = 90

o; rare monoclinic polymorphs exist

Crystal Habit: prismatic crystals with prism {011} and pyramid {111} and {113}faces; massive

crystal aggregates and encrustations are common, but also occurs in colloform (reniform) and

stalactitic aggregates


Hardness: 1½ - 2½; soft


Cleavage/Fracture: cleavage indistinct; conchoidal fracture to irregular fracture (in

masses)


Colors: bright yellow to yellowish-brown

Streak: pale yellow to white

Luster: resinous to greasy

Other: alters to sulfate minerals such as gypsum and anhydrite

Diagnostic Properties: the bright yellow color, lack of distinct cleavage and resinous to

greasy luster are distinctive, as is the commonly pale yellow streak


Refractive Indices: α = 1.96, β = 2.04, γ = 2.25

Relief: very high-extreme (+)

Color/Pleochroism: pale yellow to yellowish gray; very weakly pleochroic

Birefringence: extreme (0.287)

Twinning: usually absent

Optic Sign: Biaxial (+); 2Vz = 69o (moderately large)

Diagnostic Properties: the combination of very high to extreme relief, pale yellow

colors, absence of cleavage, very high-extreme birefringence and biaxial (+) optics with

large 2V are characteristic

Occurrences/Associations: Sulfur is a common mineral in areas of active volcanism where it

forms encrustations and mounds around fumuaroles and hot springs. The sulfur forms by

sublimation from gases or by bacterial reduction of sulfate minerals precipitated from solution.

Abundant sulfur is also associated with marine evaporate deposits over salt domes. The sulfur is

concentrated between the calcium sulfate (gypsum and anhydrite) cap rocks from which it is

produced by bacterial processes and overlying calcite layers. In simple terms, bacterial reduction

of the sulfate minerals releases hydrogen sulfide gas and calcium. Oxidation of the hydrogen

sulfide by groundwater produces the sulfur and released calcium combines with carbonate ion

from groundwater to form the calcite. Less commonly, sulfur is produced by the oxidation of

sulfide minerals, e.g. in the oxidized portion sulfide veins.

Uses: Sulfur is used in the production of sulfuric acid. Most of the sulfur used for acid

production is a byproduct of the refining of sulfide ores. Sulfur is also used to aid the production

of insecticides, paper, paints, dyes and textiles.

SYLVITE:

Composition: KCl; halide mineral; isostructural with halite; but very limited solid solution


o, γ

= 90o

Crystal Habit: equant, cubic crystals {001}; in massive to granular aggregates


Hardness: 2½

Specific Gravity: 1.99; low to very low

Cleavage/Fracture: 3 orientations at 90o; cubic {001}


Colors: colorless to white when pure; also gray, blue, yellow, orange or red

Streak: white

Luster: vitreous; cloudy on partially dissolved surfaces

Other: has a slightly bitter salty taste; sectile, so doesn’t powder easily

Diagnostic Properties: its bitter taste and sectility distinguish sylvite from halite which

has similar properties and occurrence



Relief: moderate, negative; one of only a few reasonably common minerals with

moderate negative relief; due to high solubility, only occurs in thin-sections produced

without using water




Diagnostic Properties: again, must be prepared using techniques that do not readily

dissolve it; if used, the perfect cubic cleavage, isotropic nature, extremely low relief and

association with other evaporite minerals serve to distinguish it; halite is similar, but has

much higher refractive indices and very low positive relief

Occurrences/Associations: Sylvite is a relatively scarce “bitter salt” evaporite mineral produced

largely by severe desiccation during the evaporation of a restricted sea in a warm, dry climate. It

forms with other rare evaporite minerals only after more than 98% of the water has been

evaporated. Associated minerals include common evaporite minerals such as halite, gypsum and

anhydrite and rarer ones such as polyhalite, kainite and carnellite. Carbonates, clays and iron

oxide minerals are other common associates.

Uses: The major use of sylvite is as a source of potassium, essential to plant growth, in

fertilizers. It is also used as a source for potassium in a variety of industrial applications and as a

replacement for halite in the diet of those who need to restrict sodium

TALC:

Composition: Mg3Si4O10(OH)2; a phyllosilicate mineral with a T-O-T structure; rather small

amounts of substitution occur: aluminum (Al+3

) for silicon (Si+4


), ferrous

iron (Fe+2

) or ferric iron (Fe+3


); the iron-rich equivalent of talc is

minnesotaite Fe2Si4O10(OH)2

Crystal System: Triclinic (ī): a = 5.29Å, b = 9.17Å, c = 9.46Å; α = 90.4o, β = 99

o, γ = 90.1

o;

some talc crystallizes in the monoclinic system with similar dimensions for the a- and b-axes

and the c-axes roughly twice as long because the unit cell is two T-O-T layers thick, rather than

one, and with α and γ = 90o

Crystal Habit: individual crystals are scaly or “shreddy” to tabular, less commonly acicular-

capillary; occur most commonly in foliated or massive (called steatite or soapstone) crystal

aggregates; also radiating and fibrous aggregates


Hardness: 1; extremely soft


Cleavage/Fracture: one orientation of basal pinacoidal {001} cleavage; perfect; not

generally discernible in finely crystalline aggregates


Colors: typically white to light to dark green; also gray, brown, lavendar

Streak: white

Luster: typically pearly to greasy; may be dull in some finely crystalline aggregates

Other: possess a greasy-waxy feel due to extreme softness; sectile; thin sheets are

flexible, but not elastic

Diagnostic Properties: the combination of extremely low hardness, greasy-waxy feel

and luster, white-green colors and foliated habit distinguish talc from all minerals except

pyrophyllite; talc’s occurrence in magnesian metamorphic and hydrothermal associations

rather than pelitic metamorphic and hydrothermal associations permits it to be

distinguished from pyrophyllite


Refractive Indices: nα = 1.538 – l.554; nβ = 1.575 – 1.599; nγ = 1.575 – 1.602; increase

with iron content


Color/Pleochroism: colorless; minnesotaite may show pale yellow and green

pleochroism

Birefringence: 0.040 – 0.45 in sections in which α is parallel to the stage; very high; 3rd

order maximum colors common; 0.03 – 0.05; first order grays and whites in fragments

lying on basal cleavage (as in many grain mounts)

Twinning: none

Optic Sign: Biaxial (–); 2Vx = 5 - 30o; small 2V

Diagnostic Properties: the lack of color, low to moderate relief, scaly, “shreddy” to

capillary crystals, very high birefringence, and association with other magnesium-rich

minerals in metamorphosed ultramafic/ultrabasic, mafic/basic and calcareous rocks are

characteristic; phlogopite possesses higher refractive indices and relief and

displays the birdseye extinction common to micas; brucite is biaxial (+) and commonly

displays abnormal interference colors; the clay mineral sepiolite has negative relief and

considerably lower birefringence; pyrophyllite and muscovite possess a larger 2V and

occur in very different associations, e.g., pelitic metamorphic rocks

Occurrences/Associations: Talc is a common product of the hydrothermal metamorphism of

ultramafic/ultrabasic and mafic/basic igneous rocks, e.g., near oceanic ridges, where it is

associated with serpentine, brucite, magnesite and pseudomorphs of olivine and pyroxene. Talc

also occurs in relatively low- to medium-grade calcareous metamorphic rocks produced by the

metamorphism of impure dolostone where it is associated with tremolite, dolomite, calcite and

grossular-pyrope garnet. It also occurs in talc schists produced by low-grade regional

metamorphism of mafic/basic and ultramafic/ultrabasic igneous rocks during greenschist facies

metamorphism. Minnesotaite is common in metamorphosed iron formations.

Use: Talc is an important industrial mineral with many applications. One primary use is in the

production talcum powders such as baby powder, foot powders and body powders and cosmetics

including body lotions, facial creams and lipsticks. It is also used in the manufacture of

industrial lubricants, in caulking and roofing compounds and in the manufacture of paint,

plastics, rubber and paper. Talc is used in ceramic products including floor tiles, pottery and

porcelain products. Soapstone (steatite) is commonly used to carve ornamental objects.

TENNANTITE: see TETRAHEDRITE

TETRAHEDITE:

Composition: Cu12Sb4S13; a sulfosalt mineral; forms complete solid solution series with the

isostructural mineral tennantite (Cu12As4S13); much substitution of other metals, including

silver (Ag) for copper(Cu) and iron (Fe) is common leading to variable compositions in both

minerals

Crystal System: Isometric (4bar3m): a = 10.23 – 10.55Ǻ, b = 10.23 – 10.55Ǻ, c = 10.23 –

10.55Ǻ; α = 90o, β = 90

o, γ = 90

o

Crystal Habit: as the name suggests, tetrahedral crystals are common; also scarcer dodecahedra,

octahedra and tristetrahedra; as disseminated crystals and crystal groups; also as massive to

granular aggregates





Diaphaneity: opaque or nearly so

Colors: gray to gray black

Streak: black to brownish black

Luster: metallic to slightly submetallic; almost silky

Other: important ore of silver

Diagnostic Properties: The gray tetrahedral crystals are distinctive and the brownish-

black streak, moderate hardness and silky metallic luster characteristic

Microscopic Properties: opaque; gray with olive green or brownish tinge in reflected light

Occurrences/Associations: Tetrahedrite (and tennantite) occurs primarily in low to moderate

temperature hydrothermal vein and replacement deposits where it is associated with galena,

sphalerite, chalcopyrite, bornite, pyrite and a variety of silver-bearing minerals. Less commonly,

it is found in higher temperature hydrothermal vein and contact metamorphic deposits.

Uses: Tetrahedrite is an important ore of both copper and silver. Copper is used primarily in

electrical wiring and in the manufacture of alloys such as brass (with zinc) and bronze (with tin

and zinc) used for various implements. Silver is used in jewelry and in electrical components in

computers. In addition, silver has achieved significant value as a monetary hedge against

inflation and political unrest.

TITANITE: (formerly called SPHENE)

Composition: CaTiO(SiO4); a nesosilicate mineral; limited coupled substitution of (Fe+3

or Al+3

for Ti+4

) coupled with substitution of (OH-1

or F-1

for O-2

) occurs; small amounts of rare earth

elements and actinides such as uranium (U+2

) and thorium (Th+2

) may substitute for calcium

(Ca+2

)


o, γ =

90o

Crystal Habit: individual crystals are wedge-shaped, with thick tabular to bladed habits and 6-

sided, diamond-shaped basal cross sections; typically as disseminated crystals or grains; also in

lamellar to massive crystal aggregates


Hardness: 5 –5 ½; hard

Specific Gravity: 3.48 – 3.60; moderate +

Cleavage/Fracture: exists, but not generally discernible; prominent parting

Diaphaneity: slightly, to somewhat translucent

Colors: typically brown or gray; also yellow, green, and black

Streak: white

Luster: adamantine to resinous

Other: typically alters to leucoxene, a mixture of titanium oxides and other minerals

Diagnostic Properties: the hard, wedge-shaped crystals with diamond-shaped cross-

sections and the adamantine-resinous luster are characteristic



Relief: very high to extremely high, positive

Color/Pleochroism: typically shades of pale gray brown or yellow brown; may be

colorless; may be slightly pleochroic in shades of greenish yellow, yellow brown, yellow

green, pinkish, red orange and red brown

Birefringence: 0.100 – 0.192; extremely high; upper order faded “creamy white” colors,

often obscured by color and high relief

Twinning: simple twins are common; lamellar twins less so

Optic Sign: Biaxial (+); 2Vz = 17 – 40o; small to moderate 2V

Other: basal cross-sections display symmetrical extinction with the long diagonal

parallel to the fast ray; commonly zoned; may display abnormal interference colors;

Diagnostic Properties: the combination of very high relief, brownish colors (if present),

6-sided, diamond-shaped basal sections with twinning, wedge-shaped longitudinal

sections, extreme birefringence and biaxial (+) optics with small to moderate 2V are

characteristic; monazite is similar and commonly twinned, but is typically pale yellow,

has lower relief, a different crystal form, somewhat lower birefringence and a higher 2V;

zircon and xenotime are tetragonal and uniaxial and xenotime tends to be quite

pleochroic in thin-section, whereas zircon is colorless

Occurrences/Associations: Titanite is a fairly common accessory mineral in felsic and

intermediate plutonic igneous rocks including pegmatite, granitoids and diorite and in volcanic

equivalents such as rhyolite and dacite. Titanite also occurs in regional metamorphic rocks such

as schist and gneiss and in contact metamorphic rocks including skarn. It is fairly resistant to

decomposition during weathering and so is a member of the heavy mineral population in detrital

sediments.

Uses: Titanite is a minor source of titanium (Ti) which is used with iron to strengthen steel and

in alloys with aluminum, molybdenum and vanadium used in aircraft engines, missiles and


boats. Because it is physiologically inert and can integrate with bones, titanium is used in

material for joint replacements and tooth implants. Because of its light weight and strength,

titanium is used in alloys for golf clubs and eyeglass frames. Titanium oxide is used extensively

as a pigmenting agent in white paints and plastics. Some fine quality yellow-green (chartreuse)

crystals have been used as semiprecious gemstones. Although less important than zircon, titanite

(sphene) is also used in geochronology for determining rock ages using uranium-lead dating

techniques.

TOPAZ:

Composition: Al2SiO4•(F,OH)2; a nesosilicate (orthosilicate) mineral

Crystal System: Orthorhombic (2/m): a = 4.65Å, b = 8.80Å, c = 8.39Å; α = 90o, β = 90

o, γ =

90o

Crystal Habit: typically in small prismatic crystals, sometimes with cross-sections with crude

eight-sided diamond shapes (from two sets of, sometimes striated, prism faces); often with

dipyramidal terminations; usually as disseminated crystals and grains or in divergent aggregates;

also in massive to granular aggregates


Hardness: 8; very hard


Cleavage/Fracture: one orientation of basal pinacoidal {001} cleavage; perfect


Colors: variable; usually colorless to gray or wine yellow or golden brown; may also be

brown, pink, red, green, blue or violet

Streak: white


Diagnostic Properties: Distinguished from quartz by 8-sided crystal form and excellent

single cleavage orientation, as well as superior hardness and specific gravity



increasing with fluorine content

Relief: moderate


Birefringence: 0.008 – 0.011; low; maximum first order yellow

Twinning: rarely observed

Optic Sign: Biaxial (+); 2Vz = 44-68o; moderate to high

Other: 8-sided basal cross-sections possess symmetrical extinction; longitudinal sections

display parallel extinction and basal cleavage traces; cleavage traces are length fast

Diagnostic Properties: the combination of lack of color, moderate relief, excellent

cleavage (which is length fast), low birefringence, and biaxial (+) optics with moderate to

large 2V are characteristic; no other common mineral has similar characteristics

Occurrences/Associations: Topaz forms chiefly in felsic volcanic and plutonic igneous rocks.

Excellent crystals occur in cavities in rhyolite and dacite and in in pegmatite and granitoids.

Topaz also occurs in hydrothermal veins and replacement deposits and contact metamorphic

rocks with wolframite, scheelite, molybdenite, cassiterite, gold, beryl, apatite, tourmaline and

fluorite. Regionally metamorphosed pelitic rocks such as schists and gneisses may also contain

topaz.

Uses: Topaz is mined as a gemstone with various colors, most commonly wine yellow to golden

brown, but also colorless, pink, blue and violet. Topaz is also used sparingly as an abrasive in

many products, including scouring pads, sanding products, knife sharpeners and grinding

equipment.

TOURMALINE:

Composition: (Na,Ca)(Mg,Fe,Mn,Li,Al)3(Al,Fe,Mg)6(BO3)3Si6O18(OH,F,O)4; a cyclosilicate

mineral; composition can vary widely; three end-member compositions are recognized: dravite

[NaMg3Al6(BO3)3Si6O18(OH,F,O)4], schorl [NaFe3Al6(BO3)3Si6O18(OH,F,O)4], elbaite

[Na(Li,Al)3Al6(BO3)3Si6O18(OH,F,O)4]; a fourth end member is sometimes recognized: uvite

[CaMg3Al6(BO3)3Si6O18(OH,F,O)4]; there is considerable substitution solid solution between

end-members; many additional substitutions occur, as tourmaline compositions are very complex

Crystal System: Hexagonal (Trigonal): (3m): a1 = 1.594 – 1.599Å, a2 = 1.594 – 1.599Å, c =

7.19 – 7,23Å; α = 90 o, β = 120

o, γ = 90

o

Crystal Habit: typically occurs as striated prismatic crystals with crudely triangular cross-

sections; rhombohedral terminations common; in divergent to radiating crystal aggregates; also

occurs in massive aggregates and as disseminated crystals


Hardness: 7; hard – very hard


Cleavage/Fracture: none discernible; conchoidal fracture

Diaphaneity: dravite is somewhat translucent; shorl is barely translucent; elbaite is

transparent to quite translucent

Colors: highly variable; dravite is typically brown, schorl is typically black and elbaite

is typically green, pink, red, yellow or blue

Streak: white to medium gray

Luster: vitreous

Other: tourmaline is piezoelectric

Diagnostic Properties: the striated, trigonal prismatic (slender to stubby) crystals are

characteristic and the hardness, conchoidal fracture and colors are helpful in the

identification of tourmaline



Relief: moderate to high, positive

Color/Pleochroism: can be strongly pleochroic; elbaite is colorless to pale colored;

dravite and uvite are colorless to light yellow and dark yellow brown; schorl displays

deeper shades of blue, blue-green, green, olive, purple, light brown, gray and even black

Birefringence: 0.015 – 0.035; low to moderate; upper first to upper second order colors;

masked in strongly colored varieties such as schorl


Other: longitudinal sections are length fast and possess parallel extinction; color zoning

about the c-axis is common

Diagnostic Properties: moderate to high relief, lack of cleavage, roughly triangular, 3-

to 6-sided basal cross-sections, length fast longitudinal sections with parallel extinction,

common zoning and uniaxial (–) optics are characteristic; colors and birefringence vary

with composition, but the deep pleochroism of schorl is characteristic and the coloring in

dravite-uvite helpful; some biotite and hornblende may show similar relief and

pleochroism, but both have well-defined cleavage, while biotite has slightly angular,

“birdseye” extinction and is biaxial (-) and hornblende is biaxial (+) with a substantial

2V; apatite may resemble some elbaite, but has very low birefringence

Occurrences/Associations: Tourmaline is a common accessory mineral in igneous rocks

including granitoids and pegmatite. In pegmatite, it is most commonly shorl, associated with

quartz, muscovite, albite and K-feldspars; in some pegmatite, fine, gem quality elbaite crystals

also occur in association with lepidolite, beryl, apatite, spodumene, amblygonite and fluorite.

Tourmaline is also a common accessory mineral in pelitic regional metamorphic rocks such

including phyllite, schist and gneiss. It also occurs in skarns in contact metamorphic zones

adjacent to granitoids, chiefly as dravite. Tourmaline is quite resistant to weathering and so is a

common constituent of the heavy mineral fraction in detrital sediments.

Uses: Because of its piezoelectric properties, tourmaline is used in pressure gauges,

seismometers and other electronic instruments. Exceptional elbaite crystals are used as

gemstones including pink-red rubellite, blue indicolite, green Brazilian emerald and green and

red “watermelon” tourmaline.

TREMOLITE: (see ACTINOLITE)

Composition: Ca2Mg5Si8O22•(OH)2; double-chain inosilicate; an amphibole group mineral;

forms a solid solution series with actinolite which has 10-50% iron (Fe+2

) substituting for

magnesium (Mg+2

) and ferro-actinolite which has more than 50% iron (Fe+2

) substituting for

magnesium (Mg+2

). The maximum amount of Fe+2

that substitutes for Mg+2

is about 80%; small

amounts of sodium (Na+1

) may substitute for calcium (Ca+2

) coupled typically with substitution

of ferric iron (Fe+3


); small amounts of aluminum (Al+3

) substitute for

silicon (Si+4

) in a similar way; other small-scale substitutions occur

Crystal System: Monoclinic (2/m): a = 9.87Ǻ, b = 18.05Ǻ, c = 5.29Ǻ; α = 90o, β = 104.8

o, γ = 90

o

Crystal Habit: prismatic to acicular crystals in fibrous to radiated aggregates; also granular

aggregates; both types of aggregates may occur together


Hardness: 5-6


Cleavage/Fracture: two orientations of prismatic cleavage {110} not at right angles (56o

and 124o); very good, but not distinct in many aggregates

Diaphaneity: moderately translucent

Colors: typically white to gray; may be pale green

Streak: white

Luster: vitreous to pearly; silky in fibrous aggregates

Other: the pale color, hardness, habit and amphibole cleavage are diagnostic



increasing with iron content

Relief: moderate; positive


Birefringence: 0.22 – 0.27; moderate; maximum colors are 2nd

order blue and green

Twinning: simple paired twins occur

Optic Sign: Biaxial (–); 2V = 80-88o; very large 2V

Other: basal cross-sections typically 4-sided, unusual for amphiboles, and display the

two cleavages not at right angles and symmetrical extinction; longitudinal sections

typically length slow, display one cleavage and have maximum extinction angles of 12o –

20o

Diagnostic Properties: the combination of lack of color, moderate relief, good cleavage

(with 2 sets visible in basal sections), moderate birefringence, common simple twins,

length slow prismatic sections, relatively small extinction angles and biaxial (–) optics

with very high 2V are characteristic; colorless actinolite is similar, but most actinolite

displays pale pleochroism in shapes of yellow and green and has a different occurrence

and associations; hornblende displays marked pleochroism, a smaller 2V and larger

extinction angles; anthophyllite is orthorhombic, so has parallel extinction; wollastonite

possesses different cleavage and a lower 2V

Occurrences/Associations: Tremolite occurs primarily in low- to medium-grade calcareous

metamorphic rocks including (a) non-foliated impure, dolomitic marbles and calcsilicate rocks of

the albite epidote and hornblende hornfels facies and (b) in calcareous schists and impure

marbles of the greenschist and amphibolites facies. In both occurrences it is commonly

associated with calcite, dolomite, vesuvianite, talc, garnet, and epidote group minerals and less

commonly with phlogopite, diopside and even forsterite.

Uses: Fibrous varieties of tremolite have been used as a substitute for chrysotile asbestos which

was long used in fireproofing and insulation.

TRIDYMITE:


sodium (Na+1

), potassium (K+1

) and (Ca+2



) for silicon (Si+4

); occurs in two polytypes, both stable at

relatively low pressure: orthorhombic α – tridymite (low tridymite) and hexagonal β – tridymite

(high tridymite); α – tridymite (low tridymite) is the stable form in relatively low-moderate

temperature environments (< 870oC); β – tridymite (high tridymite) inverts readily through non-

displacive transformation into α – tridymite (low tridymite) on cooling; the characteristics of α –

tridymite (low tridymite) are therefore cited below; tridymite is polymorphic with other silica

group minerals including quartz and cristobalite; rarer polymorphs include the high pressure

silica minerals coesite and stichovite which occur in meteorite impactites and likely in the deep

mantle

Crystal System: Orthorhombic (222): a = 9.9Å, b = 17.1Å, c = 16.3Å; α = 90o, β = 90

o, γ =

90o; monoclinic and triclinic varieties of α – tridymite have been reported

Crystal Habit: typically occurs as tabular to platy crystals with hexagonal cross-sections; also as

acicular to capillary crystals in radiating to stellated (spheroidal) aggregates; also in granular



Hardness: 7; hard to very hard


Cleavage/Fracture: none; conchoidal fracture; may not be evident

Diaphaneity: transparent to moderately translucent

Colors: colorless to white or gray

Streak: white

Luster: vitreous

Other: α–quartz (low quartz) pseudomorphs after hexagonal β–tridymite (high

tridymite) occur, although the inversion is displacive, so the kinetics are slow

Diagnostic Properties: tridymite is difficult to distinguish from other macrocrystalline

varieties of silica such as quartz, without optical or XRD methods; the tabular crystals are

suggestive, the biaxial character of α – tridymite (low tridymite) and lower indices of

refraction are distinctive (see below)





Birefringence: 0.002 – 0.004; quite low; maximum first order grays

Twinning: three wedge-shaped sets of twins are common, thus the name

Optic Sign: Biaxial (+); 2Vz = 40 – 90o; typically 70

o ± 10

o; high to very high 2V

Diagnostic Properties: the combination of color, moderate negative relief, lack of

cleavage, wedge-shaped twins, very low birefringence and biaxial (+) optics with

generally high 2V are characteristic; cristobalite has a different crystal form, lower relief

and is uniaxial (–) and nearly isotropic; in microcrystalline varieties, the distinction is

less clear and may require XRD or other analytical methods

Occurrences/Associations: Tridymite is a relatively scarce mineral that occurs primarily in

silica-oversaturated, felsic and intermediate volcanic and hypabyssal rocks, especially rhyolite,

rhyodacite and dacite as phenocrysts, as part of the groundmass or in spherulites formed by the

divitrification of glass. It also lines vesicles in felsic, intermediate and even mafic/basic volcanic

rocks, commonly in the form of amygdules. Tridymite has been reported from some stony

meteorites.

Use: Tridymite has no economic value.

TURQUOISE:

Composition: CuAl6(PO4)4(OH)8•4H2O; a complete solid solution series exists with the rare end

member chalcosiderite: CuFe6(PO4)4(OH)8•4H2O; but the amount of iron (Fe) is usually small

Crystal System: Triclinic ( i ): a = 7.48Ǻ , b = 9.95Ǻ, c = 7.69Ǻ; α = 112o, β = 115

o, γ = 69

o

Crystal Habit: mostly cryptocrystalline or very tiny crystals; typically occurs in

cryptocrystalline, massive aggregates or as disseminated grains; also as veins, encrustations and

in colloform (reniform to stalactictic) aggregates




Cleavage/Fracture: one orientation of pinacoidal {001} cleavage; perfect; a second

orientation not at 90o of pinacoidal cleavage {010} is good; neither cleavage is

commonly discerned


Colors: typically bluish green; also sky blue and light green

Streak: white or pale green

Luster: subvitreous; waxy to porcelain-like

Diagnostic Characteristics: Turquoise can be distinguished from most other minerals by

its blue-green to sky blue color, hardness and porcelain-like luster; chrysocolla is much

softer; malachite and azurite are softer, have different colors and effervesce readily in

HCl.



Relief: moderate

Color/Pleochroism: pale shades of blue, blue-green or green

Birefringence: 0.04; low; first order grays

Optic Sign: Biaxial (+); 2Vz = 40o; moderate

Diagnostic Properties: the combination of pale blue, blue-green to green color,

moderate relief, low birefringence and biaxial (+) optics with moderate 2V is

characteristic

Occurrences/Associations: Turquoise occurs mostly as a product of the hydrothermal alteration

of volcanic rocks such as rhyolite, dacite and trachyte, associated with copper mineralization.

Uses: Turquoise is widely used as a gemstone in jewelry and decorative objects.

UVAROVITE: see GARNET

ULEXITE:

Composition: NaCaB5O6•5H2O; a borate mineral

Crystal System: Triclinic (ī): a = 8.71Ǻ, b = 12.72Ǻ, c = 6.69Ǻ; α = 90.3o, β = 109

o, γ = 105

o

Crystal Habit: commonly as acicular to filiform crystals which occur in fibrous aggregates; also

in loose “cotton-ball” aggregates, nodular masses and encrustations


Hardness: 2½; soft


Cleavage/Fracture: one orientation; pinacoidal {100}; perfect; there are two much less

discernible orientations not at 90o; prismatic {1ī0}; fair

Diaphaneity: somewhat translucent to quite translucent

Colors: colorless to white or light gray

Streak: white

Luster: silky in fibrous aggregates; otherwise vitreous to subvitreous

Other: “television rock” consists of closely packed, parallel fibers of ulexite

Diagnostic Characteristics: distinguished most easily by its acicular-filiform crystals in

soft, “cotton balls” or fibrous habit





Birefringence: 0.029 – 0.32; high; up to lower 3rd

order colors

Twinning: simple and polysynthetic twins occur

Optic Sign: Biaxial (+); 2Vx = 73o; high

Diagnostic Properties: the combination of lack of color, one perfect cleavage, low-

moderate negative relief, common acicular-fibrous habit, high birefringence and biaxial

(+) optic sign with large 2V and association with other continental evaporites are

characteristic; borax is rarely fibrous, has lower birefringence and a lower 2V

Occurrences/Associations: Most ulexite forms by evaporation in enclosed saline lakes in warm,

arid climates and as an efflorescent formed by soil water evaporation under similar conditions.

Commonly associated minerals include bornite, kernite, colemanite, gypsum, anhydrite, halite

and a host of rare borate minerals.

Uses: The most important use of ulexite is in production of borax for the manufacture of glass

fibers for use in insulation and textiles. Borax is also used in detergent soaps and as a flux in

metallurgical processes such as welding and smelting. One isotope of elemental boron, obtained

from ulexite, is used in the shields of atomic reactors and elemental boron is also used in fuels

for rockets and automobiles and in the high strength plastics used in aircraft manufacturing.

URANINITE:

Composition: UO2; an oxide mineral; forms a complete solid solution series with thorianite

(ThO2), so usually contains substantial thorium; often contains lead (Pb) isotopes and helium

(He) produced by radioactive decay of uranium (and thorium).

Crystal System: Isometric (4/mbar3/m): a1 = 5.47Ǻ, a2 = 5.47Ǻ, a3 = 5.47Ǻ; α = 90o, β = 90

o,

γ = 90o

Crystal Habit: individual crystals are scarce equant octahedra {111}, sometimes with cubic

{001} dodecahedral {011} modifying faces; occurs commonly as “pitchblende” in massive to

granular aggregates or in colloform (botryoidal-reniform) encrustations of radiating acicular-

capillary crystals with a banded structure


Hardness: 5½; hard

Specific Gravity: 10.9 when pure; usually 6.5 – 10.0; very high to extremely high



Colors: brownish to steely black

Streak: brownish black to grayish

Luster: submetallic to “pitchy”; earthy in fine-grained aggregates

Other: highly radioactive; alters to yellow carnotite, green autunite and many other

uranium-bearing minerals

Diagnostic Properties: The black color, brownish-black streak, pitchy luster and

extremely high specific gravity are distinctive; the radioactivity is the clincher

Microscopic Properties: nearly opaque; brownish gray in reflected light

Occurrences/Associations: Uraninite is a generally scarce accessory mineral that forms mostly

in medium to high temperature (mesothermal to hypothermal) veins where it is associated with

chalcopyrite, cassiterite, arsenopyrite and cobaltite. Uraninite is also concentrated in detrital

sediments by precipitation from solution during the reduction of pore waters during bacterial

decomposition.

Uses: Uraninite is the principle ore of uranium (Ur) used as a fuel in nuclear power generation

and in thermonuclear bombs.

VESUVIANITE: formerly called idocrase

Composition: Ca10(Mg,Fe)2Al4(SiO4)5(Si2O7)2(OH)4; a sorosilicate mineral; composition is

quite variable due to substantial substitution of (Na+1

for Ca+2

) (Mn+2

, Fe+3

for Mg+2

and Fe+2

)

(Fe+3

for Al+3

) and (Al+3

for Si+4

); sparse amounts of other elements may substitute as well

Crystal System: Tetragonal (4/m2/m2/m): a1 = 1.550Å, a2 = 1.550Å, c = 11.80Å; α = 90o, β =

90o, γ = 90

o

Crystal Habit: commonly striated, nearly equant to stubby prismatic crystals with pinacoidal

{001} terminations; also as radiating prismatic crystals; as disseminated crystals; less commonly

in massive to granular aggregates; rarely acicular-fibrous




Cleavage/Fracture: imperfect; none discernible


Colors: variable; commonly shades of brown, green or yellow; also red and blue

Streak: white


Diagnostic Properties: the stubby striated prismatic crystals, brown to green color and

hardness are characteristic; zircon tetragonal and brown, but is harder and its crystals

typically have dipyramidal truncations



Relief: high to very high

Color/Pleochroism: colorless to often pale shades of gray, brown, yellow-brown, yellow

green, even red brown; not pleochroic

Birefringence: 0.001 – 0.015; very low to low; somewhat obscured by color

Optic Sign: Uniaxial (–); some biaxial forms reported

Other: square basal cross-sections common; longitudinal sections are length fast,

commonly displays abnormal interference colors in shades of Berlin blue and green;

may be zoned

Diagnostic Properties: the combination of no color or pale brown shades, high relief,

square cross-sections, low birefringence, common abnormal interference colors and

uniaxial (–) length fast optics is characteristic, as is its occurrence in calcareous

metamorphic rocks; clinozoisite resembles colorless vesuvianite, but it has a perfect

cleavage, is biaxial (–) and has a different occurrence and associations; epidote resembles

colored vesuvianite, but typically possesses higher birefringence, has angular extinction

and is biaxial (–) with a substantial 2V

Occurrences/Associations: Vesuvianite occurs primarily in low to medium grade contact and

regional metamorphic calcareous rocks such as skarn, marble and calcsilicate marble. It is

common in rocks of the albite-epidote hornfels, hornblende hornfels, greenschist, epidote

amphibolite and amphibolite facies where it is associated with minerals that include calcite,

dolomite, graphite, phlogopite, grossularite garnet, diopside, tremolite and wollastonite.

Uses: Vesuvianite does not generally possess economic value, although transparent specimens

are sometimes used as minor gemstones.

WAD: see manganite; technically a rock or soil, but commonly discussed with minerals

Composition: Variable; Wad is a finely crystalline mixture of different hydroxide,

oxyhydroxide and oxide minerals including romanechite [Ba,H2O)2Mn5O10], todorokite

[(Mn,Ca.Mg)Mn3O7•H2O], pyrolusite (MnO2), manganite (MnOOH), cryptomelane

(KMn8O16) and other manganese minerals with clay minerals; analogous to bauxite for

aluminum hydroxides and oxyhydroxides

Crystal System: not applicable

Crystal Habit: typically occurs in fine-grained, earthy masses and as surficial encrustations;

forms nodular masses and layers in bogs (bog manganese) and on the ocean floor (manganese

nodules); forms dendrites on fracture surfaces (joints and faults); contributes to black surface

stains (rock varnish) on rock surfaces, especially in deserts (desert varnish)


Hardness: 1 – 6½; soft to hard; variable depending on mineralogy and compactness

(porosity) of earthy and nodular masses

Specific Gravity: 3 - 7; moderate very high; variable depending on mineralogy and

compactness (porosity) of earthy and nodular masses

Cleavage/Fracture: none discernible

Diaphaneity: generally opaque

Colors: typically gray black to black

Streak: brownish black to black

Luster: dull in earthy masses; submetallic in nodular masses

Diagnostic Properties: combination of black color, brownish black to black streak,

earthy to submetallic luster is distinctive; nodular masses are distinctive

Microscopic Properties: see manganite; opaque

Occurrences/Associations: Wad is formed by the alteration of manganese bearing minerals in

soils, by precipitation on ocean floors (“manganese nodules”) and in bogs (bog manganese) and

also accumulates in a variety of other near surface environments.

Uses: Wad, especially, in the form of nodules, is the major source of manganese (Mn) which is

primarily used as a hardening agent in the manufacture of steel for support beams, automobiles

and machinery. It is also used as a hardening agent with aluminum and copper, e.g., in the

production of electrical wires and transmission lines. Manganese is also use as a coloring agent

in the production of glassware and ceramics. It is used in the production of potassium

permanganate (KMnO4), used to kill bacteria and algae in water and wastewater treatment.

WITHERITE:

Composition: BaCO3; a carbonate; isostructural with aragonite, strontianite and cerrusite, all

members of the orthorhombic (aragonite) group of carbonate minerals; limited solid solution

with strontianite (Sr+2

substitutes for Ba+2

)


90o, γ = 90

o

Crystal Habit: penetration twins nearly always produce stubby prismatic dipyramidal

pseudohexagonal crystals with horizontal striations; often in colloform (botryoidal, globular)

aggregates; also in massive to granular aggregates; acicular crystals in fibrous aggregates also

occur




Cleavage/Fracture: one cleavage; pinacoidal {010} fair; others poor so all cleavages are

difficult to discern; subconchoidal fracture


Colors: colorless to white; also light gray to yellow-brown

Streak: white

Luster: vitreous


barite

Diagnostic Characteristics: witherite can be distinguished from aragonite by its higher

specific gravity; it most closely resembles strontianite (which has a lower specific

gravity) from which it can be distinguished, in large crystals, by the latter’s two

orientations of cleavage and more clearly by a test for strontium


Refractive Indices: nα= 1.529; nβ = 1.676; nγ = 1.677

Relief: low negative to moderate-high positive; sections in which the alpha vibration

direction is visible produce a “twinkling” effect as the stage is rotated under plane light


Birefringence: 0.148; very high-extreme; high order faded colors (“creamy white”)


Optic Sign: Biaxial (–); 2Vx = 16o (small 2V)

Other: longitudinal sections are length slow with parallel extinction



uniaxial (–); strontianite is similar, but it is a more common mineral, with much better

cleavage, that possesses slightly lower refractive indices and relief and 2V, and possesses

a markedly lower specific gravity; aragonite has a higher 2Vand higher refractive indices;

cerrusite is distinguished by its very high refractive indices and relief and the lack of

“twinkling” as the stage is rotated under plane light

Occurrences/Associations: Most witherite occurs in low-temperature hydrothermal veins and

replacement products in limestone where it is associated with galena, sphalerite, fluorite and

barite.

Uses: a minor ore of barium used to increase the density drilling muds used in the recovery of

petroleum from wells which helps to prevent “blowouts”; also used in the manufacture of glass,

paper and ceramics products and in the production of rubber and plastics; for x-ray images of

the stomach and intestinal tract

WOLFRAMITE:

Composition: (Fe,Mn)WO4; a complete solid solution series exists between MnWO4

(huebnerite) and FeWO4 (ferberite)

Crystal System: Monoclinic (2/m): a = 4.77, b = 5.71, c = 4.98; α = 90o, β = 91

o, γ = 90

o; so

nearly orthorhombic

Crystal Habit: tabular-bladed crystals with prominent front pinacoid {100}; divergent to

lamellar aggregates; also massive to granular aggregates


Hardness: 4 – 4½

Specific Gravity: 7.0 – 7.5; very high; higher with increasing iron (Fe) content

Cleavage/Fracture: one cleavage orientation; pinacoidal {010}

Diaphaneity: barely translucent to nearly opaque

Colors: dark brown to black (in iron-rich varieties)

Streak: dark brown to nearly black

Luster: submetallic to resinous

Diagnostic Characteristics: The dark brown to black color and streak, very high specific

gravity and one orientation of cleavage are distinctive.


Refractive Indices: nα = 2.17 - 2.31; nβ = 2.22 – 2.40; nγ = 2.30 – 2.46


Color/Pleochroism: pleochroic in shades of yellow, orange, green and red brown

Birefringence: 0.13 – 0.15; low; upper first order colors, but obscured by pleochroic

colors

Twinning: common simple contact twins

Optic Sign: biaxial (+); 2Vz = 73o – 89

o; high to very high

Diagnostic Properties: the strong pleochroism in shades of yellow-green-orange-red

brown, the extreme relief and relatively low birefringence are suggestive; the common

twinning, biaxial (+) optic sign and very high 2V are characteristic

Occurrences/Associations: Wolframite occurs in granitic pegmatites and from high-temperature

hydrothermal vein and replacement deposits where it is associated with cassiterite, scheelite,

molybdenite, arsenopyrite, topaz, apatite, pyrite and quartz.

Uses: Wolframite is the major ore of tungsten (W) used primarily to harden steel for the

manufacture of tools used for cutting and drilling, especially power tools. Tungsten is also used

in the manufacture of silicon carbide, an abrasive harder than any mineral except diamond,

which is used wherever hard substances are required. Pure tungsten has long been used for

filaments in lamps and light bulbs.

WOLLASTONTE:

Composition: CaSiO3; single-chain inosilicate; a pyroxenoid mineral with rotated single chains;

small amounts of manganese (Mn+2

) or iron (Fe+2

) and less magnesium (Mg+2

), may substitute

for calcium (Ca+2

)


o, γ = 103

o

Crystal Habit: individual crystals may be acicular, tabular or bladed; commonly occurs in

fibrous or granular aggregates, often within the same specimen; may occur in foliated aggregates


Hardness: 4 ½ – 5 ½; moderate to hard


Cleavage/Fracture: one orientation of pinacoidal {100} cleavage; perfect; two others,

orientations, one near 90o (84

o) to the prominent cleavage and the other not at right angles

(70o) to the prominent cleavage; good only; the combination tends to produce coarse,

splintery fracture


Colors: commonly white; also light gray to pale green

Streak: white

Luster: vitreous to pearly; silky in fibrous aggregates

Other: commonly alters to calcite

Diagnostic Properties: distinguished from tremolite with difficulty unless in cleavable

masses where the splintery cleavage with two orientations near 90o and often lower

hardness distinguish it from tremolite



Relief: moderate to moderate plus, positive


Birefringence: 0.013 – 0.017; low plus; maximum colors are first order yellow to red

Twinning: simple paired twins common

Optic Sign: Biaxial (–); 2Vx = 36 – 60o; moderate 2V

Other: two orientations of cleavage visible in most sections, sometimes near right angles,

sometimes not; extinction may be somewhat mottled, so not all parts of the crystal go to

extinction at the same time

Diagnostic Properties: the combination of lack of color, moderate relief, good

cleavage, rather low birefringence, common twinning and biaxial (–) optics with

moderate 2V are characteristic, as is the occurrence in calcareous metamorphic rocks;

tremolite possesses typical amphibole cleavage (2 sets at 56o and 124

o) and a larger 2V;

pectolite is more typically radial-fibrous and has smaller extinction angles and higher

birefringence, as well as a different association

Occurrences/Associations: Wollastonite is an abundant mineral in medium to high grade

calcareous metamorphic rocks such as marble, skarn, and calcsilicate rocks produced by the

metamorphic transformation of impure limestone and dolostone. It is produced by contact

metamorphism the hornblende hornfels and pyroxene hornfels facies and by regional

metamorphism in the amphibolite and granulite facies. Commonly associated minerals include

calcite, dolomite, diopside, forsterite, grossular garnet and calcic plagioclase, and less commonly

tremolite, vesuvianite and epidote. Wollastonite has been reported from rare alkalic igneous

rocks.

Uses: Fibrous wollastonite is used as a healthier replacement for asbestos used to produce fire-

resistant construction materials such as insulation, construction board, siding and roofing

materials. Wollastonite possesses properties such as resistance to alteration, flexural strength

and durability that make it useful in the production of high quality ceramics, as an additive to

paints and in the production of plastics and rubber products.

WULFENITE:

Composition: PbMoO4; a molybdate mineral

Crystal System: Tetragonal (4): a1 = 5.44Ǻ, a2 = 5.44Ǻ, c = 12.11Ǻ; α = 90o, β = 90

o, γ = 90

o

Crystal Habit: rather thin, tabular crystals with square cross-sections; sometimes with pyramidal

or dipyramidal (where twinned) faces; commonly in reticulated aggregates


Hardness: 3; soft to moderate transition

Specific Gravity: 6-5 – 7.0; very high

Cleavage/Fracture: 2 orientations not at 90o; good

Diaphaneity: somewhat translucent to transparent

Colors: typically yellow-orange-red; also white to gray

Streak: white


Diagnostic Characteristics: the square, thin, tabular crystals and yellow-orange-red

color are characteristic as is its very bright luster and low hardness; the rare lead

chromate mineral Crocoite (PbCrO4) can only be distinguished with certainty by

chemical analysis




Color/Pleochroism: pleochroic in shades of yellow to pale orange

Birefringence: 0.122; extremely high; high order faded colors (“creamy white”)

Twinning: contact twins common


Diagnostic Properties: the combination of yellow-orange pleochroism, square to

rectangular sections, extreme relief and birefringence and uniaxial (–) optics are

characteristic

Occurrences/Associations: Wulfenite occurs in the oxidized zone of galena-bearing

hydrothermal vein and replacement deposits. It is formed by the reaction of acidic rich, meteoric

ground water with galena (PbS) and molybdenite (MoS2) to cause oxidation. What wulfenite is

associated with depends on the other primary sulfide minerals, but common associates include

cerussite, anglesite, limonite (goethite), pyromorphite, and vanadinite.

Uses: The crystals are prized for their bright yellow-orange-red colors. Wulfenite is a minor ore

of molybdenum (Mo) used in the manufacture of steel alloys.

ZIRCON:

Composition: ZrSiO4; a nesosilicate (orthosilicate) mineral; some hafnium (Hf+4

) typically

substitutes for zirconium (Zr+4

) in the range of 1-5%, as do small amounts of rare earth elements

and actinides (uranium and thorium); metamict zircon has a structure that has been largely

destroyed by alpha decay of radioactive uranium and thorium; limited amounts of phosphate

(PO4)-4

may substitute for silica (SiO4)-4

Crystal System: Tetragonal (4/m2/m2/m): a1 = 6.61Å, a2 = 6.61Å, c = 5.98Å; α = 90o, β =

90o, γ = 90

o

Crystal Habit: typically small prismatic crystals; with tetragonal prisms {110} and tetragonal

dipyramids {111}; typically as disseminated crystals and grains


Hardness: 7½; very hard


Cleavage/Fracture: generally not discernible


Colors: typically brown, yellow brown or reddish brown; also gray, pink, blue-green

Streak: white

Luster: vitreous to sub-adamantine; greasy

Diagnostic Properties: The small tetragonal prismatic crystals with dipyramidal

terminations, its elevated hardness and the commonly brownish color are characteristic


Refractive Indices: nω = 1.920 – 1.960; nε = 1.967 – 2.015; increases with hafnium

content; less in metamict zircon (~1.800)

Relief: very high to extremely high, positive; less in metamict zircon

Color/Pleochroism: colorless to pale brown, but obscured in small crystals by extreme

relief

Birefringence: 0.036 – 0.065; high to very high; maximum third or fourth order colors;

but essentially isotropic in metamict crystals

Optic Sign: Uniaxial (+); metamict zircon is essentially isotropic

Other: longitudinal sections are length slow, with parallel extinction; crystals commonly

zoned; where zircon crystals occur as inclusions in other minerals alpha decay

commonly produces a pleochroic “halo” in the mineral that surrounds the inclusions

Diagnostic Properties: the combination of pale brown to no color, very high relief and

birefringence, lack of cleavage and uniaxial (+), length slow optics is characteristic;

xenotime resembles zircon, but is usually pleochroic and has significantly lower relief

and higher birefringence; titanite is superficially similar, but possesses higher

birefringence, six-sided diamond-shaped basal sections and wedge-shaped longitudinal

sections and is biaxial (+); monazite is biaxial with a small 2V and angular extinction

Occurrences/Associations: Zircon is a widely distributed accessory mineral formed in felsic

rocks such as granitoids, syenite, monzonite, foid syenite and pegmatite. It also occurs in

metamorphic rocks, including marble, schist and gneiss. Zircon is extremely resistant to

chemical decomposition during weathering and diagenesis, so is commonly concentrated in the

heavy mineral population of detrital sediments where it can form commercially valuable placer

deposits.

Uses: The mineral zircon is utilized in foundry sand used to make castings and used in the form

of zirconium oxide in the production of refractory bricks. High quality zircon is also used as a

gemstone, with colorless specimens used as “substitutes” for diamond. Zircon is also the

principal source for zirconium (Zr) and Hafnium (Hf) and a secondary source for many rare earth

elements (REE). Zirconium is used in nuclear reactors and to make the synthetic stone “cubic

zirconia” which is widely used in jewelry. Zircon is also widely used to precisely date

geological events using uranium-lead dating techniques. Hafnium is used to absorb neutrons in

nuclear power plant control rods, in superalloys for jet engines and its oxide form is used in the

fabrication of integrated circuits for computers.

ZOISITE:

Composition: Ca2Al3O(SiO4)(Si2O7)(OH); sorosilicate; epidote group mineral; forms a partial

solid solution series with clinozoisite with less iron (Fe+3

) substituting for (Al+3

); clinozoisite

contains more (Fe+3

) substituting for (Al+3

) and is therefore monoclinic; still higher amounts of

iron (Fe+3

) result in the monoclinic mineral epidote; where a few percent of manganese (Mn+2

)

substitutes in zoisite, the pink zoisite called thulite results


90o, γ = 90

o

Crystal Habit: acicular to striated prismatic (or bladed) crystals occur chiefly as disseminated

crystals or in granular to aggregates; also radiating aggregates




Cleavage/Fracture: one orientation of pinacoidal {010} cleavage; perfect;


Colors: variable; typically greenish gray to greenish brown; also white, gray, blue-violet

or pink to rose red (thulite)

Streak: white


Diagnostic Properties: the grayish green color and single excellent cleavage distinguish

zoisite from most amphiboles (two cleavages); the color distinguishes it from most

clinozoisite and epidote which are deeper green due to their higher iron content



with increasing iron

Relief: moderate-high

Color/Pleochroism: colorless; except thulite which is pleochroic in shades of pale pink

to yellow

Birefringence: 0.005 – 0.020; low to moderate; maximum colors are first order grays to

first order red-purple

Twinning: none

Optic Sign: Biaxial (+); 2Vx = 0 – 50o; very low to moderate 2V

Other: longitudinal sections display parallel extinction and are either length slow or

length fast; zoning is common; some zoisite and thulite display abnormal interference

colors in blue and brown;

Diagnostic Properties: the lack of color, moderately-high relief, excellent pinacoidal

cleavage, low-moderate birefringence, parallel extinction, abnormal interference colors

and biaxial (+) optics with low-moderate 2V are characteristic; clinozoisite possesses

angular extinction and slightly higher relief and birefringence; epidote is often pleochroic

in shades of yellow and green, has inclined extinction and much higher birefringence;

vesuvianite is uniaxial (+); thulite is distinguished from other zoisite by its pink to

yellow pleochroism; piemontite has similar colors, but has angular extinction and higher

birefringence

Occurrences/Associations: Zoisite occurs mostly in medium-grade (amphibolite facies)

calcareous metamorphic rocks, including marble, impure marble and calcsilicate rocks, and in

mafic rocks including amphibolite. It is also reported from high pressure metamorphic rocks of

the blueschist and eclogite facies.

Uses: Zoisite is not an economically important mineral. Transparent specimens of good color

are used as minor gemstones.

Documents

Final Mineral Descriptions