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A Tour of the Rock Forming Silicates

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Common SilicatesGroup Igneous Metamorphic Sedimentary

Neso (SiO4) Olivine group, Zircon, Topaz

Garnet group, Alumnosilicates

Soro (Si2O7) Epidote Epidote, Zoesite

Cyclo (SiO3) Tourmaline

Ino (single – SiO3

to Si2O6)Pyroxene group (Mg, Fe, Ca)

Pyroxene group (Ca, Na)

Ino (double – Si4O11)

Amphibole (hornblende)

Amphibole

Phyllo (Si4O10) Mica (biotite-muscovite)

Mica, Chlorite, Serpentine, Talc

Clays

Tecto (SiO2) Quartz, Feldspar Quartz, Feldspar, Zeolite

Quartz, Feldspar, Zeolite

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Structural Formulas and Silicates

• The key to understanding silicate mineral groups, solid solution, and miscibility

• Symbology– W = large cations, C.N. >6 (with oxygen)

• Ca, Na, K– X = medium-sized, bivalent cations, C.N. = 6 (with

oxygen) • Mg, Fe+2 , & Ca (sort of)

– Y = medium-sized, trivalent cation, C.N. = 6 (with oxygen)

• Typically Al and sometimes Fe+3

– Z = small cations, C.N. = 4 (with oxygen) • Mainly Si+4, but also Al+3

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Structural Formulas

• Olivine Group– Nesosilicates [SiO4]

– X2SiO4

– X = Mg, Fe

• Pyroxene Group– Single chain inosilicates

[SiO3 or Si2O6]

– X2SiO3 to (W,X,Y)2Z2O6

• Feldspar Group– Tecto (framework) silicates

[SiO2]

– WZ4O8

• Garnet Group– Nesosilicates [SiO4]

– X3Y2 (SiO4)3

– X = Ca, Mn, Fe, Mg– Y = Fe+3, Cr

• Amphibole Group– Double chain inosilicates

[Si8O22]

– W0-1X2Y5(Z8O22)(OH, F)2

• Mica Group– Phylo (sheet) silicates

[Si4O10]

– W(X,Y)2-3(Z4O10)(OH, F)2

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Nesosilicates: Olivine Group• Common

component in ultramafic-mafic igneous rocks (peridotite, gabbro, basalt)

• A complete solid solution series– X2SiO4

– X = Mg, Fe– 2:1 metals:Si

Flattened crystals of Fayalite

Peridot (Forsterite)

Dunite xenolith

Dunite inclusion in basalt

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• Complete solid solution:– Forsterite: Mg2SiO4

– Fayalite: Fe2SiO4

– Also CaMg and CaFe end members, very rare

• Mg end member (Fo) crystallizes first– Mg slightly more

compatible than Fe

• Crystallize at high T

Nesosilicates: Olivine Group

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• Garnet Group (X3Y2 {SiO4}3)– X{A} –> divalent cations: Ca+2, Mn+2, Fe+2, Mg+2

– Y{B} –> trivalent cations: Al+3, Fe+3, Cr+3

– Generally high-grade (high temp and/or pressure) metamorphic rock occurrence

– Gem stone of high hardness: 7-7.5

Nesosilicates: Garnet Group

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• Isomorphous minerals with some solid solution

– Pyralspites(Mg, Mn, Fe)

• Pyrope• Almandine• Spessartine

– Ugrandites(Ca)

• Uvarovite • Grossular• Andradite

uvarovite

Nesosilicates: Garnet Group

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• Accessory minerals in rocks– Zircon (ZrSiO4)

• Commonly contains uranium and thorium (and daughter product: lead) as minor atomic substitution components

• Highly useful for geochronology: radiometric dating using the unstable isotopes U & Th -> Pb

• Also common as an accessory mineral in metamorphic and sedimentary (highly resistant) rocks

– Topaz (Al2SiO4 {OH,F}2)• Pegmatite/hydrothermal accessory

mineral• Hard a gem mineral

Nesosilicates: Other

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Sorosilicates and Cyclosilicates• Sorosilicate: Epidote

Group– Common in regional

metamorphism (epidote-ampibolite facies)

– Isostructural group with some solid solution

• Cyclosilicate: Tourmaline– Common component in

pegmatite– Hosts incompatible elements– Used as gemstone

Tourmaline (“watermelon”)

Epidote

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Inosilicates: Pyroxene Group

• Common in mafic igneous & metamorphic rocks

• Typically: X2Si2O6– X is the divalent metal

cations: Mg, Fe, (Ca)– Enstitite (Mg), Ferrosilite

(Fe), Diopside (CaMg), Hedenburgite (CaFe)

– 1:1 metals:Si• Some amount of Ca is

also possible in the Pyroxene structure– Wollastonite (Ca): not a

pyroxene, rather a related mineral called a pyroxenoid

• Single Chain Inosilicate

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• The Pyroxene Quadrilateral

• “Real” pyroxenes– Clinopyroxenes

• Diopside• Hedenburgite• AUGITE

– Orthopyroxenes• Enstitite• Ferrosilite• HYPERSTHENE

Inosilicates: Pyroxene Group

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• Pyroxene quadrilateral and miscibility gaps

Inosilicates: Pyroxene Group

• Gap between ortho- and clinopyroxene– Accommodation

of Ca

• Gap varies by temperature

• Ca deficient clinopyroxene:– Augite

• Ca rich orthopyroxene:– Pigeonite (hi-T

only)

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• Coupled Substitution– Divalent/trivalent substitutions in tetrahedral (CN 4) and

octahedral (CN 6) sites of many rock forming silicate minerals

– Substitution of cations with different valences• Ex: Na+ for Ca2+

– Maintaining overall charge balance requires additional substitutions

• Ex Na+ and Si4+ for Ca2+ and Al3+

• In pyroxenes: Sodic pyrobole species– Coupled substitution of Na+Al3+ for 2Ca2+

– Jadeite->NaAlSi2O6

– High pressure metamorphic minerals

Inosilicates: Pyroxene Group

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Chemistry Clinopyroxene

CaMg Diopside (Cpx)

Ca(MgFe) Augite (Cpx)

CaFe Hedenburgite (Cpx )

Some Sodium Rich Varieties:

NaFe Aegirine

NaAl Jadeite (the gem)

LiAl Spodumene

• General pyroxene formula: (W,X,Y)2Z2O6

Inosilicates: Pyroxene Group

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Inosilicates: Amphibole Group• Double chain, hydrous

silicates – W0-1X2Y5(Z8O22)(OH)2

– W = Na, K– X = Ca, Na, Mn, Fe, Mg, Li– Y = Mn, Fe, Mg, Fe3+, Al, Ti– Z = Al, Si

• Closely related to pyroxenes– Same cations; amphiboles

have water– Complete and partial solid

solution– Coupled substitution– Orthorhombic and monoclinic

Ferro-actinolite

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Inosilicates: Amphibole Group

• Solid solution relationships and miscibility gap analogous to pyroxenes

Ferro-actinolite

• Mg-Fe complete solution series

• CaMg-CaFe solution series

• Miscibility gap due to accommodation of Ca

• NaAl coupled substitution series

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• Hornblende: the most common (and a complicated) amphibole

• “Any black amphibole”• Typical in intermediate igneous rocks• Also common high temperature metamorphic rocks

Hornblende

(K,Na)0-1(Ca,Na,Fe,Mg)2 (Mg,Fe,Al)5(Si,Al)8O22(OH)2

Inosilicates: Amphibole Group

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Phylosilicates: Mica Group• Natural micas

– K(Al,Mg,Fe)2-

3(AlSi3O10)(OH)2

– Muscovite(Al micas)

– Biotite(Fe-Mg micas)

• Miscibility gap between biotite series and muscovite

• Hydrous sheet silicate

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• Essential minerals in– Igneous rocks

• Muscovite: Felsic igneous rocks, Granites

• Biotite: Felsic to intermediate rocks

– Metamorphic rocks• Schists

Biotite

Phylosilicates: Mica Group

pseudo-hexagonal crystalline aggregate of muscovite

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• Serpentine– Mg3Si2O5(OH)4

– Low-grade alteration of olivine, pyroxene, and amphibole

• Talc– Mg3Si4O10(OH)2

– Low-grade metamorphic rocks

• Chlorite– (Mg,Fe)3(Si,Al)4O10(OH)2 *

(Mg,Fe)3(OH)6

– Greenschist facies metamorphic rocks

Phylosilicates: OtherSerpentine

Talc

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Tectosilicates: Feldspar Group• The most abundant minerals in the Earth’s crust

• Framework silicates (SiO2)

– WZ4O8

• (Ca,Na,K)(Al,Si)4O8

– Plagioclase Series• Anorthite to albite

• (Ca,Na)(Al,Si)4O8

– Alkali Feldspars• Orthoclase to albite

• (Na,K)(Al,Si)4O8

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Plagioclase Series• Essential minerals in most

igneous, sedimentary, and metamorphic rocks

• Complete (temperature dependant) solid solution between

– Albite (NaAlSi3O8)

– Anorthite (CaAl2Si2O8)

– Minor solid solution of K+ increasing with increasing Ab content

Tectosilicates: Feldspar Group

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• Plagioclase series: complete solid solution

Tectosilicates: Feldspar Group

– Because of the similarity in ionic radius between Na+1 (0.95 Å) and Ca+2 (0.99 Å)

– Isomorphous solid solution between end members

• Coupled substitution– (CaxNa1-x)(Al1+xSi3-x)O8

– Ex: Ca.20Na.80(Al1.2Si2.8)O8

• Oligoclase

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• Alkali Feldspar: Miscibility gap– Because of dissimilarity in size between the Na+1 (0.95 Å) and K+1

(1.33 Å) complete solid solution occurs only at high temp

• Alkali feldspar polymorphs

• Single alkali feldspar crystal formed at high temp exsolves at lower temp (if slowly cooled)– Orthoclase-rich-> perthite– Albite-rich-> antiperthite

Tectosilicates: Feldspar Group

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Tectosilicates: Quartz Group

• Essentially “pure” SiO2

• Component of many felsic and intermediate igneous rocks– Not present in:

• Ultramafic igneous rocks• Alkaline (feldspathoidal) igneous

rocks

• Common particulate residue during bedrock weathering • Common chemical

precipitate in surface through hydrothermal settings

• Common component of metamorphic rocks

Amethyst

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• Silica P-T phase diagram– Silica

polymorphs in P-T “space”

– Hi P: coesite, stishovite

– Hi T: tridymite, cristobalite

Tectosilicates: Quartz Group

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• Chalcedony: a micro- (very small) to crypto- crystalline (almost amorphous {non-crystalline}) fibrous quartz

• Common precipitate in surface and near-surface conditions

Tectosilicates: Quartz Group

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Minerals and Rocks

Formation Essential minerals

Silicate Non-silicate

Igneous Crystallize from magma

Olivine, pyroxene, amphibole, mica, feldspar, quartz

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Sedimentary Weathering residue

Chemical precipitate

Quartz, feldspar, clayQuartz (microcrystalline)

--

Carbonates

Halides

Metamorphic Alteration of pre-existing mineral

Garnet, epidote, pyroxene, amphibole, mica, quartz, feldspar

Carbonates