Introduction to the Silicate Minerals

Earth Materials – EPSc 352

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The culprit: [SiO4]4- tetrahedron

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Mineral StructuresMinerals are classified on the basis of Si-O polymerism

Nesosilicates

[SiO4]4- Isolated tetrahedra

Examples: olivine, garnet

Sorosilicates

[Si2O7]6- Double tetrahedra

Examples: lawsonite, epidote

Cyclosilicates

n[SiO3]2- n = 3, 4, 6

Examples: benitoite BaTi[Si3O9]

axinite Ca3Al2BO3[Si4O12]OH

beryl Be3Al2[Si6O18] 39/24/2019

Mineral StructuresMinerals are classified on the basis of Si-O polymerism

Inosilicates

[SiO3]2- [Si4O11]

4-

Single chains Double chains

pyroxenes/pyroxenoids amphiboles49/24/2019

Mineral StructuresMinerals are classified on the basis of Si-O polymerism

Phyllosilicates

[Si2O5]2- sheets of tetrahedra

Examples: mica, talc, clays, serpentine

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Mineral StructuresMinerals are classified on the basis of Si-O polymerism

Tectosilicates

[SiO2]0

3D framework of tetrahedra, fully polymerized

Examples: quartz, feldspars, feldspathoids, zeolites69/24/2019

NesosilicatesIsolated tetrahedra

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NesosilicatesIsolated tetrahedra

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NesosilicatesIsolated tetrahedra

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Garnet

X3Y2(SiO4)3

NesosilicatesIsolated tetrahedra

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SorosilicatesDouble tetrahedra

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Hemimorphite

Zn4Si2O7(OH)2·H2O

CyclosilicatesDouble tetrahedra

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Beryl

Be3Al2Si6O18

InosilicatesChains of tetrahedra

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Augite (Pyroxene)

(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6

InosilicatesChains of tetrahedra

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Hornblende (Amphibole)

Ca2(Mg, Fe, Al)5 (Al, Si)8O22(OH)2

InosilicatesChains of tetrahedra

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Cleavage angles can be interpreted in terms of weak bonds in M2

sites (around I-beams instead of through them)

Narrow single-chain I-beams → 90o cleavages in pyroxenes while wider

double-chain I-beams → 60-120o cleavages in amphiboles

PhyllosilicatesSheets of tetrahedra

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Muscovite

KAl2(AlSi3O10)(F,OH)2

Serpentine

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Octahedra are a bit larger than

tetrahedral match, so they cause

bending of the T-O layers (after

Klein and Hurlbut, 1999).

Antigorite maintains a

sheet-like form by

alternating segments of

opposite curvature

Chrysotile does not do

this and tends to roll into

tubes

Tectosilicates3D framework of tetrahedra

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AlbiteNaAlSi3O8

Tectosilicates3D framework of tetrahedra

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Quartz Polymorphs

Tectosilicates3D framework of tetrahedra

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Quartz Cristobalite

Tridymite Coesite

Streak

Density

Color

Hardness

Cleavage

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Identification of SilicatesHand Sample

• Color of the mineral when it’s powdered

• Not helpful for most silicates

• Powder is usually white

• Some silicate minerals are harder than streak plate

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Identification of SilicatesStreak

• Usually darker-colored silicates are denser (this is not always true)

• e.g., garnet • Use a few silicate minerals of known density

for comparison purposes

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Identification of SilicatesDensity

• Be careful about use of color

• Only useful in some specific cases (e.g., specific shades of green – olivine，epidote)

• For Fe-Mg silicates (e.g., olivine), the darker the color, the more Fe in it

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Identification of SilicatesColor

• Silicate minerals are mostly harder than non-silicates, but very few of them are harder than quartz

• Quartz hardness = 7

• Kyanite: Al2SiO5, interesting example• Displays greatly different hardness properties along

different directions

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Identification of SilicatesHardness

• Silicate minerals are mostly harder than non-silicates, but very few of them are harder than quartz

• Quartz hardness = 7

• Kyanite: Al2SiO5, interesting example• Displays greatly different hardness properties along

different directions

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Identification of SilicatesHardness

• Cleavage is defined as the tendency of a crystal to break along a plane in certain definite directions

• A good feature to use to distinguish among the silicates (e.g., amphibole and pyroxene)

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Identification of SilicatesHardness

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Presentation inspired by slides from:• Molly C. McCanta (University of Tennessee, Knoxville)• John D. Winter (Whitman College, now retired)