Mineral Structures

Mineral StructuresMineral StructuresSilicates are classified on the basis of Si-O polymerism Silicates are classified on the basis of Si-O polymerism

the [SiOthe [SiO44]]4-4- tetrahedron tetrahedron

Mineral StructuresMineral StructuresSilicates are classified on the basis of Si-O polymerism Silicates are classified on the basis of Si-O polymerism

[SiO[SiO44]]4-4- Independent tetrahedra Independent tetrahedra NesosilicatesNesosilicates

Examples: olivine garnetExamples: olivine garnet

[Si[Si22OO77]]6-6- Double tetrahedra Double tetrahedra SorosilicatesSorosilicates

Examples: lawsonite epidoteExamples: lawsonite epidote

n[SiOn[SiO33]]2-2- n = 3, 4, 6 n = 3, 4, 6 CyclosilicatesCyclosilicates

Examples: benitoite BaTi[SiExamples: benitoite BaTi[Si33OO99]]

beryl Beberyl Be33AlAl22[Si[Si66OO1818]]

Mineral StructuresMineral StructuresInosilicates Inosilicates

[SiO[SiO33]]2-2- single chains single chains Inosilicates Inosilicates [Si[Si44OO1111]]4-4- Double chains Double chains

pryoxenes pyroxenoidspryoxenes pyroxenoids amphiboles amphiboles

Mineral StructuresMineral StructuresPhyllosilicates Phyllosilicates

[Si[Si22OO55]]2-2- Sheets of tetrahedra Sheets of tetrahedra PhyllosilicatesPhyllosilicates

micas talc clay minerals serpentinemicas talc clay minerals serpentine

Mineral StructuresMineral StructuresTectosilcates Tectosilcates

[SiO[SiO22] 3-D frameworks of tetrahedra: fully polymerized ] 3-D frameworks of tetrahedra: fully polymerized TectosilicatesTectosilicates

quartz and the silica minerals feldspars feldspathoids zeolitesquartz and the silica minerals feldspars feldspathoids zeolites

low-quartzlow-quartz

Nesosilicates: independent SiONesosilicates: independent SiO44 tetrahedra tetrahedra

Olivine (100) view blue = M1 yellow = M2Olivine (100) view blue = M1 yellow = M2

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M1 and M2 as polyhedraM1 and M2 as polyhedra

Nesosilicates: Olivine (Mg,Fe)Nesosilicates: Olivine (Mg,Fe)22SiOSiO44

Olivine Occurrences:Olivine Occurrences: Principally in mafic and ultramafic igneous rocks- Principally in mafic and ultramafic igneous rocks-

Typically ~60+% of mantle source for basaltsTypically ~60+% of mantle source for basalts-- Fayalite in meta-ironstones and in some alkalic Fayalite in meta-ironstones and in some alkalic

granitoidsgranitoids Forsterite in some siliceous dolomitic marblesForsterite in some siliceous dolomitic marbles

Nesosilicates: Garnet Nesosilicates: Garnet

Garnet (001) view blue = Si purple = A turquoise = BGarnet (001) view blue = Si purple = A turquoise = B

Garnet: AGarnet: A2+2+33 B B3+3+

22 [SiO [SiO44]]3 3

““Pyralspites”Pyralspites” - B = Al - B = AlPyPyrope: Mgrope: Mg33 Al Al22 [SiO [SiO44]]3 3

AlAlmandine: Femandine: Fe33 Al Al22 [SiO [SiO44]]33

SpSpessartine: Mnessartine: Mn33 Al Al22 [SiO [SiO44]]33

““Ugrandites”Ugrandites” - A = Ca - A = CaUUvarovite: Cavarovite: Ca33 Cr Cr22 [SiO [SiO44]]33

GrGrossularite: Caossularite: Ca33 Al Al22 [SiO [SiO44]]33

AndAndradite: Caradite: Ca33 Fe Fe22 [SiO [SiO44]]33

Occurrence:Occurrence:Mostly metamorphicMostly metamorphicSome high-Al igneousSome high-Al igneousAlso in some mantle peridotitesAlso in some mantle peridotites

Inosilicates: single chains- Inosilicates: single chains- pyroxenespyroxenes

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

Diopside: CaMg [SiDiopside: CaMg [Si22OO66]]

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a si

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sin

Where are the Si-O-Si-O chains??Where are the Si-O-Si-O chains??

Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

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a si

na

sin

The tetrahedral chain The tetrahedral chain above the M1s is above the M1s is

offset from that below offset from that below

The result is a The result is a monoclinicmonoclinic unit cell, unit cell, hence hence clinopyroxenesclinopyroxenes

e.g. Diopside, Augitee.g. Diopside, Augite


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(+) M1(+) M1

(+) M2(+) M2

(+) M2(+) M2

OrthopyroxeneOrthopyroxene

an an orthorhombicorthorhombic unit unit cellcell

Enstatite (MgEnstatite (Mg22SiSi22OO66))


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(+) M1(+) M1

(-) M1(-) M1

(-) M2(-) M2

(+) M2(+) M2

Pyroxene ChemistryPyroxene Chemistry

The general pyroxene formula: The general pyroxene formula:

WW1-P1-P (X,Y) (X,Y)1+P1+P Z Z22OO66

WhereWhere W = W = CaCa Na Na X = X = Mg FeMg Fe2+2+ Mn Ni Li Mn Ni Li Y = Al FeY = Al Fe3+3+ Cr Ti Cr Ti Z = Z = SiSi Al Al

Anhydrous Anhydrous so high-temperature or dry conditions so high-temperature or dry conditions favor pyroxenes over amphibolesfavor pyroxenes over amphiboles


The pyroxene quadrilateral and opx-cpx solvusThe pyroxene quadrilateral and opx-cpx solvusCoexisting opx + cpx in many rocks (pigeonite only in volcanics)Coexisting opx + cpx in many rocks (pigeonite only in volcanics)

DiopsideDiopside HedenbergiteHedenbergite

WollastoniteWollastonite

EnstatiteEnstatite FerrosiliteFerrosiliteorthopyroxenes

clinopyroxenes

pigeonite (Mg,Fe)(Mg,Fe)22SiSi22OO66 Ca(Mg,Fe)SiCa(Mg,Fe)Si22OO66

pigeonite clinopyroxenes

orthopyroxenes

SolvusSolvus

12001200ooCC

10001000ooCC

800800ooCC


““Non-quad” pyroxenesNon-quad” pyroxenesJadeiteJadeite

NaAlSiNaAlSi22OO66

Ca(Mg,Fe)SiCa(Mg,Fe)Si22OO66

AegirineAegirine

NaFeNaFe3+3+SiSi22OO66

Diopside-HedenbergiteDiopside-Hedenbergite

Ca-Tschermack’s Ca-Tschermack’s moleculemolecule CaAlCaAl22SiOSiO66

Ca / (Ca + Na)Ca / (Ca + Na)

0.20.2

0.80.8

Omphaciteaegirine- augite

AugiteAugite

Inosilicates: double chains- Inosilicates: double chains- amphibolesamphiboles

Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg)Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg)yellow = M4 (Ca)yellow = M4 (Ca)

Tremolite:Tremolite:CaCa22MgMg55 [Si [Si88OO2222] (OH)] (OH)22

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a si

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Inosilicates: double chains- Inosilicates: double chains- amphibolesamphiboles

Hornblende:Hornblende:(Ca, Na)(Ca, Na)2-3 2-3 (Mg, Fe, Al) (Mg, Fe, Al)55

[(Si,Al)[(Si,Al)88OO2222] (OH)] (OH)22

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a si

na

sin

Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)

little turquoise ball = Hlittle turquoise ball = H

General formula:General formula:

WW0-10-1 X X22 Y Y55 [Z [Z88OO2222] (OH, F, Cl)] (OH, F, Cl)22

W = Na KW = Na K

X = Ca Na Mg FeX = Ca Na Mg Fe2+2+ (Mn Li) (Mn Li)

Y = Mg FeY = Mg Fe2+2+ Mn Al Fe Mn Al Fe3+3+ Ti Ti

Z = Si AlZ = Si Al

Again, the great variety of sites and sizes Again, the great variety of sites and sizes a great chemical range, and a great chemical range, and hence a broad stability rangehence a broad stability range

The The hydroushydrous nature implies an upper temperature stability limit nature implies an upper temperature stability limit

Amphibole ChemistryAmphibole Chemistry

Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes)Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes)


TremoliteTremoliteCaCa22MgMg55SiSi88OO2222(OH)(OH)22

FerroactinoliteFerroactinoliteCaCa22FeFe55SiSi88OO2222(OH)(OH)22

AnthophylliteAnthophyllite

MgMg77SiSi88OO2222(OH)(OH)22FeFe77SiSi88OO2222(OH)(OH)22

Actinolite

Cummingtonite-grunerite

OrthoamphibolesOrthoamphiboles

ClinoamphibolesClinoamphiboles

Hornblende has Al in the tetrahedral siteHornblende has Al in the tetrahedral site

Geologists traditionally use the term “hornblende” as a catch-all term for practically Geologists traditionally use the term “hornblende” as a catch-all term for practically any dark amphibole. Now the common use of the microprobe has petrologists any dark amphibole. Now the common use of the microprobe has petrologists casting “hornblende” into end-member compositions and naming amphiboles casting “hornblende” into end-member compositions and naming amphiboles after a well-represented end-member.after a well-represented end-member.

Sodic amphiboles Sodic amphiboles

Glaucophane: NaGlaucophane: Na2 2 MgMg3 3 AlAl2 2 [Si[Si88OO2222] (OH)] (OH)22

Riebeckite: NaRiebeckite: Na2 2 FeFe2+2+3 3 FeFe3+3+

2 2 [Si[Si88OO2222] (OH)] (OH)22

Sodic amphiboles are commonly blue, and often called “blue amphiboles”Sodic amphiboles are commonly blue, and often called “blue amphiboles”


Tremolite (Ca-Mg) occurs in meta-carbonatesTremolite (Ca-Mg) occurs in meta-carbonates

Actinolite occurs in low-grade metamorphosed basic igneous rocksActinolite occurs in low-grade metamorphosed basic igneous rocks

The complex solid solution called hornblende occurs in a broad variety of both The complex solid solution called hornblende occurs in a broad variety of both igneous and metamorphic rocksigneous and metamorphic rocks

Sodic amphiboles are predominantly metamorphic where they are Sodic amphiboles are predominantly metamorphic where they are characteristic of high P/T subduction-zone metamorphism (commonly called characteristic of high P/T subduction-zone metamorphism (commonly called “blueschist” in reference to the predominant blue sodic amphiboles “blueschist” in reference to the predominant blue sodic amphiboles

Amphibole OccurrencesAmphibole Occurrences

InosilicatesInosilicates

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

Narrow single-chain I-beams Narrow single-chain I-beams 90 90oo cleavages in pyroxenes while wider double- cleavages in pyroxenes while wider double-chain I-beams chain I-beams 60-120 60-120oo cleavages in amphiboles cleavages in amphiboles

pyroxenepyroxene amphiboleamphibole

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SiOSiO44 tetrahedra polymerized into 2-D sheets: [Si tetrahedra polymerized into 2-D sheets: [Si22OO55]]

Apical O’s are unpolymerized and are bonded to other constituentsApical O’s are unpolymerized and are bonded to other constituents

PhyllosilicatesPhyllosilicates

Tetrahedral layers are bonded to octahedral layers Tetrahedral layers are bonded to octahedral layers

(OH) pairs are located in center of T rings where no apical O(OH) pairs are located in center of T rings where no apical O



Gibbsite: Al(OH)Gibbsite: Al(OH)33

Layers of octahedral Al in coordination with (OH)Layers of octahedral Al in coordination with (OH)

AlAl3+3+ means that means that only 2/3 of the VI sites may be occupiedonly 2/3 of the VI sites may be occupied for charge-balance reasons for charge-balance reasons

Brucite-type layers may be called Brucite-type layers may be called trioctahedraltrioctahedral and gibbsite-type and gibbsite-type dioctahedraldioctahedral

aa11

aa22


Muscovite:Muscovite: KK Al Al22 [Si [Si33AlAlOO1010] (OH)] (OH)2 2 (coupled K - Al(coupled K - AlIVIV))

T-layer - T-layer - didiocathedral (Alocathedral (Al3+3+) layer - T-layer - ) layer - T-layer - KK

T T O O T T KK T T O O T T KK T T O O TT

K between T - O - T groups is stronger than vdwK between T - O - T groups is stronger than vdw


Phlogopite:Phlogopite: K Mg K Mg33 [Si [Si33AlOAlO1010] (OH)] (OH)22

T-layer - T-layer - tritriocathedral (Mgocathedral (Mg2+2+) layer - T-layer - ) layer - T-layer - KK

T T O O T T KK T T O O T T KK T T O O TT

K between T - O - T groups is stronger than vdwK between T - O - T groups is stronger than vdw

Chlorite: (Mg, Fe)Chlorite: (Mg, Fe)33 [(Si, Al) [(Si, Al)44OO1010] (OH)] (OH)22 (Mg, Fe) (Mg, Fe)33 (OH) (OH)66

= T - O - T - (brucite) - T - O - T - (brucite) - T - O - T -= T - O - T - (brucite) - T - O - T - (brucite) - T - O - T -

Very hydrated (OH)Very hydrated (OH)88, so low-temperature stability (low-T metamorphism , so low-temperature stability (low-T metamorphism

and alteration of mafics as cool)and alteration of mafics as cool)


TectosilicatesTectosilicates

After Swamy and Saxena (1994) J. Geophys. Res., 99, 11,787-11,794.


Low Quartz StishoviteLow Quartz Stishovite

SiSiIVIV Si SiVIVI


FeldsparsFeldspars

Albite: Albite: NaNaAlAlSiSi33OO88

Substitute two Substitute two AlAl3+3+ for Si for Si4+4+ allows Caallows Ca2+2+ to to be addedbe added

Substitute AlSubstitute Al3+3+ for Sifor Si4+4+ allows allows NaNa++ or K or K++ to be to be addedadded

Mineral Structures

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