How Do We Identify How Do We Identify Minerals?Minerals?

Earth ScienceEarth Science

Ms. KurtzweilMs. Kurtzweil

Spring 2010Spring 2010

Identification by Identification by Careful Inspection…Careful Inspection…

ColorColor

► First and most easily observed mineral First and most easily observed mineral property.property.

► Some minerals have very characteristic Some minerals have very characteristic colors that help identify them..colors that help identify them.. CinnabarCinnabar MalachiteMalachite SulfurSulfur AzuriteAzurite

BUT… Color is also the least BUT… Color is also the least useful property for mineral useful property for mineral

identificationidentification► Many different minerals Many different minerals

have similar colorshave similar colors Orthoclase feldspar, Orthoclase feldspar,

calcite, others… milky calcite, others… milky whitewhite

► Traces of impurities can Traces of impurities can change a mineral’s colorchange a mineral’s color Traces of impurities can Traces of impurities can

change a mineral’s color… change a mineral’s color… Ex: Quartz / CorundumEx: Quartz / Corundum► + Fe… amethyst / +Cr… + Fe… amethyst / +Cr…

rubyruby► + Ti… rose quartz / + Co, + Ti… rose quartz / + Co,

Ti… sapphireTi… sapphire

► A mineral may change color when exposed A mineral may change color when exposed to air to air

►Weathered surfaces may hide true color…Weathered surfaces may hide true color… So always inspect freshly-exposed surfaces!So always inspect freshly-exposed surfaces!

LusterLuster

► Luster: how light is reflected from the Luster: how light is reflected from the surface of a mineralsurface of a mineral

► Are either metallic or nonmetallicAre either metallic or nonmetallic►Metallic- shines like polished metalMetallic- shines like polished metal

GalenaGalena Pyrite Pyrite

► Nonmetallic lusters:Nonmetallic lusters:► Glassy (vitreous)Glassy (vitreous)

Quartz Quartz ► PearlyPearly

Mica, talc Mica, talc ► WaxyWaxy

Sulfur, sphaleriteSulfur, sphalerite► Brilliant (adamantine)Brilliant (adamantine)

Diamond Diamond ► EarthyEarthy

Gypsum Gypsum ► Others (greasy, oily, Others (greasy, oily,

dull)dull)

Crystal ShapeCrystal Shape

► When minerals have When minerals have enough time and space enough time and space to form, their atoms or to form, their atoms or ions always arrange ions always arrange themselves into themselves into patternspatterns

► Form 1 of 6 basic Form 1 of 6 basic crystal shapescrystal shapes Flat-facedFlat-faced Regularly shapedRegularly shaped RARE to findRARE to find

Identification by Identification by Simple Tests…Simple Tests…

StreakStreak

► Streak: the color of a Streak: the color of a mineral in powdered formmineral in powdered form

► Although the color of a Although the color of a mineral may vary, the mineral may vary, the streak rarely does. Much streak rarely does. Much more reliable!more reliable!

► Examples:Examples: Iron pyrite (brass-yellow) Iron pyrite (brass-yellow)

streak: greenish-blackstreak: greenish-black Hematite (brown,red,silver) Hematite (brown,red,silver)

streak: reddish brownstreak: reddish brown

► To test streak:To test streak: Rub specimen across an unglazed ceramic tile, Rub specimen across an unglazed ceramic tile,

called a streak plate (white or black)called a streak plate (white or black)

► Streak rule of thumbStreak rule of thumb Metallics: streak will be at least as dark as hand Metallics: streak will be at least as dark as hand

specimenspecimen Nonmetallics: streak is usually colorless or whiteNonmetallics: streak is usually colorless or white

CleavageCleavage

► Cleavage: a mineral’s Cleavage: a mineral’s tendency to break tendency to break evenly or separate evenly or separate along a flat surface.along a flat surface.

► Ability is related to the Ability is related to the type of bond in atomic type of bond in atomic structure- will separate structure- will separate at weak bondsat weak bonds

► Can be observed in Can be observed in tiny grains; therefore tiny grains; therefore very usefulvery useful

►Micas (muscovite, Micas (muscovite, biotite)biotite) 1 perfect plane of 1 perfect plane of

cleavagecleavage Parallel sheetsParallel sheets

► Calcite, galenaCalcite, galena 3 planes of cleavage3 planes of cleavage Right anglesRight angles

►Fracture: when minerals with no Fracture: when minerals with no cleavage planes break unevenly into cleavage planes break unevenly into curved or irregular piecescurved or irregular pieces Uneven/irregular Uneven/irregular

►Ex. Cinnabar, some quartzEx. Cinnabar, some quartz

Splintery/fibrousSplintery/fibrous► asbestosasbestos

Conchoidal- like broken glassConchoidal- like broken glass►Obsidian Obsidian

DensityDensity

► Depends on the kind of atoms Depends on the kind of atoms present (more massive?) and present (more massive?) and how closely packed they arehow closely packed they are

► HeftHeft Galena “feels” heavier than Galena “feels” heavier than

quartzquartz► Density is more specific: Density is more specific:

d=m/vd=m/v► Most common minerals in Most common minerals in

Earth’s crust range from Earth’s crust range from ► Best for identifying heavier Best for identifying heavier

metals (Au, Pb, U, Ag) which metals (Au, Pb, U, Ag) which range fromrange from

HardnessHardness

►Hardness: the ability Hardness: the ability of a mineral to resist of a mineral to resist scratchingscratching

►Mohs’ Scale of Mohs’ Scale of HardnessHardness Used as a practical test Used as a practical test

for determining a for determining a mineral’s hardnessmineral’s hardness

1 (softest) Talc1 (softest) Talc 10 (hardest) Diamond10 (hardest) Diamond

Mohs Knows BestMohs Knows Best► Don’t confuse a streak with a scratch!Don’t confuse a streak with a scratch!

Rub away streak to feel and look for the scratchRub away streak to feel and look for the scratch► Ask:Ask:

Which is the hardest mineral on the scale that it can Which is the hardest mineral on the scale that it can scratch?scratch?

Which numbers does it fall between?Which numbers does it fall between? If neither scratch each other, they have the same If neither scratch each other, they have the same

hardnesshardness► Graphite and diamond are both pure CGraphite and diamond are both pure C

Graphite: 1, Diamond: 10Graphite: 1, Diamond: 10 Difference is in their bonds!Difference is in their bonds!

Special Properties…Special Properties…

MagnetismMagnetism

►Mineral can be Mineral can be picked up by a picked up by a magnet, or will pick magnet, or will pick up small metal up small metal objectsobjects

► Examples:Examples: MagnetiteMagnetite LodestoneLodestone

Fluorescence/Fluorescence/PhosphorescencePhosphorescence

► Fluorescence: ability Fluorescence: ability to glow under to glow under ultraviolet (UV) lightultraviolet (UV) light Some fluorite, calcite Some fluorite, calcite

samplessamples

► Phosphorescence: Phosphorescence: ability to glow after ability to glow after UV light is cut offUV light is cut off Some willemite, Some willemite,

sphalerite samplessphalerite samples

Double RefractionDouble Refraction

► Refraction: the Refraction: the bending of light rays bending of light rays as they pass through as they pass through a translucent a translucent substancesubstance

►Double refraction: the Double refraction: the splitting of light rays splitting of light rays into two parts, into two parts, causing a double causing a double imageimage

► CalciteCalcite

EffervescenceEffervescence

► A mineral is said to A mineral is said to display effervescence display effervescence if it fizzes in a if it fizzes in a chemical reaction chemical reaction with cold, weak HClwith cold, weak HCl

► Limestone and MarbleLimestone and Marble Calcite Calcite (CaCO(CaCO33) is the ) is the

principle mineralprinciple mineral Bubbles released are Bubbles released are

carbon dioxidecarbon dioxide

RadioactivityRadioactivity

► Certain atoms have unstable Certain atoms have unstable electron and/or nuclei electron and/or nuclei arrangementsarrangements

► An unstable nucleus becomes An unstable nucleus becomes stable over time by releasing stable over time by releasing particles and energy particles and energy (radiation)(radiation)

►Most common elements: Most common elements: Uranium and RadonUranium and Radon

►Minerals: carnotite and Minerals: carnotite and uraninite found in Uuraninite found in U