Mineral StructuresSilicates are classified on the basis of Si-O polymerism The culprit: the [SiO4]4- tetrahedron

Mineral StructuresSilicates are classified on the basis of Si-O polymerism [SiO4]4- Independent tetrahedra Nesosilicates

Examples: olivine garnet

[Si2O7]6- Double tetrahedra Sorosilicates

Examples: lawsonite

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

Examples: benitoite BaTi[Si3O9] axinite Ca3Al2BO3[Si4O12]OH beryl Be3Al2[Si6O18]

Mineral StructuresSilicates are classified on the basis of Si-O polymerism [SiO3]2- single chains Inosilicates [Si4O11]4- Double tetrahedrapryoxenes pyroxenoids amphiboles

Mineral StructuresSilicates are classified on the basis of Si-O polymerism [Si2O5]2- Sheets of tetrahedra Phyllosilicatesmicas talc clay minerals serpentine

Mineral StructuresSilicates are classified on the basis of Si-O polymerism [SiO2] 3-D frameworks of tetrahedra: fully polymerized Tectosilicatesquartz and the silica minerals feldspars feldspathoids zeoliteslow-quartz

Mineral StructuresNesosilicates: independent SiO4 tetrahedra

Nesosilicates: independent SiO4 tetrahedra Olivine (100) view blue = M1 yellow = M2bcprojection

Olivine (100) view blue = M1 yellow = M2bcperspectiveNesosilicates: independent SiO4 tetrahedra

Olivine (001) view blue = M1 yellow = M2M1 in rows and share edges

M2 form layers in a-c that share corners

Some M2 and M1 share edgesbaNesosilicates: independent SiO4 tetrahedra

Nesosilicates: independent SiO4 tetrahedra Olivine (100) view blue = M1 yellow = M2bcM1 and M2 as polyhedra

Nesosilicates: independent SiO4 tetrahedra

Olivine Occurrences:Principally in mafic and ultramafic igneous and meta-igneous rocks Fayalite in meta-ironstones and in some alkalic granitoidsForsterite in some siliceous dolomitic marblesMonticellite CaMgSiO4 Ca M2 (larger ion, larger site)High grade metamorphic siliceous carbonates

Nesosilicates: independent SiO4 tetrahedra Garnet (001) view blue = Si purple = A turquoise = BGarnet: A2+3 B3+2 [SiO4]3

Pyralspites - B = AlPyrope: Mg3 Al2 [SiO4]3 Almandine: Fe3 Al2 [SiO4]3 Spessartine: Mn3 Al2 [SiO4]3 Ugrandites - A = CaUvarovite: Ca3 Cr2 [SiO4]3 Grossularite: Ca3 Al2 [SiO4]3 Andradite: Ca3 Fe2 [SiO4]3

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

Nesosilicates: independent SiO4 tetrahedra Garnet (001) view blue = Si purple = A turquoise = BGarnet: A2+3 B3+2 [SiO4]3

Pyralspites - B = AlPyrope: Mg3 Al2 [SiO4]3 Almandine: Fe3 Al2 [SiO4]3 Spessartine: Mn3 Al2 [SiO4]3 Ugrandites - A = CaUvarovite: Ca3 Cr2 [SiO4]3 Grossularite: Ca3 Al2 [SiO4]3 Andradite: Ca3 Fe2 [SiO4]3

Occurrence:Mostly metamorphicPyralspites in meta-shalesUgrandites in meta-carbonatesSome high-Al igneousAlso in some mantle peridotitesa1a2a3

Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Diopside: CaMg [Si2O6] ba sinWhere are the Si-O-Si-O chains??

Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)ba sin

Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)ba sin

Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)ba sin

Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)ba sin

Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)ba sin

Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Perspective view

Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)SiO4 as polygons(and larger area)IV slabIV slabIV slabIV slabVI slabVI slabVI slabba sin

Inosilicates: single chains- pyroxenes M1 octahedron

Inosilicates: single chains- pyroxenes M1 octahedron

Inosilicates: single chains- pyroxenes M1 octahedron(+) type by convention(+)

Inosilicates: single chains- pyroxenes M1 octahedron

This is a (-) type(-)

Inosilicates: single chains- pyroxenes TM1TCreates an I-beam like unit in the structure.

Inosilicates: single chains- pyroxenes TM1TCreates an I-beam like unit in the structure

The pyroxene structure is then composed of alternating I-beamsClinopyroxenes have all I-beams oriented the same: all are (+) in this orientation Inosilicates: single chains- pyroxenes Note that M1 sites are smaller than M2 sites, since they are at the apices of the tetrahedral chains

The pyroxene structure is then composed of alternation I-beamsClinopyroxenes have all I-beams oriented the same: all are (+) in this orientation Inosilicates: single chains- pyroxenes

Tetrehedra and M1 octahedra share tetrahedral apical oxygen atoms Inosilicates: single chains- pyroxenes

The tetrahedral chain above the M1s is thus offset from that below

The M2 slabs have a similar effect

The result is a monoclinic unit cell, hence clinopyroxenesInosilicates: single chains- pyroxenes ca(+) M1(+) M2(+) M2

Orthopyroxenes have alternating (+) and (-) I-beams

the offsets thus compensate and result in an orthorhombic unit cell

This also explains the double a cell dimension and why orthopyroxenes have {210} cleavages instead of {110) as in clinopyroxenes (although both are at 90o)Inosilicates: single chains- pyroxenes ca(+) M1(-) M1(-) M2(+) M2

Pyroxene ChemistryThe general pyroxene formula: W1-P (X,Y)1+P Z2O6

WhereW = Ca NaX = Mg Fe2+ Mn Ni LiY = Al Fe3+ Cr TiZ = Si Al

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

Pyroxene ChemistryThe pyroxene quadrilateral and opx-cpx solvusCoexisting opx + cpx in many rocks (pigeonite only in volcanics)DiopsideHedenbergiteWollastoniteEnstatiteFerrosiliteorthopyroxenesclinopyroxenespigeonite(Mg,Fe)2Si2O6Ca(Mg,Fe)Si2O6pigeoniteclinopyroxenesorthopyroxenesSolvus1200oC1000oC800oC

Pyroxene ChemistryNon-quad pyroxenesJadeiteNaAlSi2O6Ca(Mg,Fe)Si2O6AegirineNaFe3+Si2O6Diopside-HedenbergiteCa-Tschermacks moleculeCaAl2SiO6Ca / (Ca + Na)0.20.8Omphaciteaegirine- augiteAugiteSpodumene: LiAlSi2O6

PyroxenoidsIdeal pyroxene chains with 5.2 A repeat (2 tetrahedra) become distorted as other cations occupy VI sitesWollastonite (Ca M1) 3-tet repeatRhodoniteMnSiO3 5-tet repeatPyroxmangite (Mn, Fe)SiO3 7-tet repeatPyroxene2-tet repeat

Inosilicates: double chains- amphiboles Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg)yellow = M4 (Ca)Tremolite:Ca2Mg5 [Si8O22] (OH)2ba sin

Inosilicates: double chains- amphiboles Hornblende:(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2ba sinHornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)little turquoise ball = H

Inosilicates: double chains- amphiboles Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe)Hornblende:(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2Same I-beam architecture, but the I-beams are fatter (double chains)

Inosilicates: double chains- amphiboles ba sinSame I-beam architecture, but the I-beams are fatter (double chains)All are (+) on clinoamphiboles and alternate in orthoamphibolesHornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)little turquoise ball = HHornblende:(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2

Inosilicates: double chains- amphiboles 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)little turquoise ball = HHornblende:(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2

M1-M3 are small sites

M4 is larger (Ca)

A-site is really big

Variety of sites great chemical range

Inosilicates: double chains- amphiboles 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)little turquoise ball = HHornblende:(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2

(OH) is in center of tetrahedral ring where O is a part of M1 and M3 octahedra(OH)

See handout for more informationGeneral formula:

W0-1 X2 Y5 [Z8O22] (OH, F, Cl)2

W = Na KX = Ca Na Mg Fe2+ (Mn Li)Y = Mg Fe2+ Mn Al Fe3+ TiZ = Si Al

Again, the great variety of sites and sizes a great chemical range, and hence a broad stability rangeThe hydrous nature implies an upper temperature stability limitAmphibole Chemistry

Ca-Mg-Fe Amphibole quadrilateral (good analogy with pyroxenes)Amphibole ChemistryAl and Na tend to stabilize the orthorhombic form in low-Ca amphiboles, so anthophyllite gedrite orthorhombic series extends to Fe-rich gedrite in more Na-Al-rich compositionsTremoliteCa2Mg5Si8O22(OH)2FerroactinoliteCa2Fe5Si8O22(OH)2AnthophylliteMg7Si8O22(OH)2Fe7Si8O22(OH)2ActinoliteCummingtonite-gruneriteOrthoamphibolesClinoamphiboles

Hornblende has Al in the tetrahedral site

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 casting hornblende into end-member compositions and naming amphiboles after a well-represented end-member.

Sodic amphiboles

Glaucophane: Na2 Mg3 Al2 [Si8O22] (OH)2Riebeckite: Na2 Fe2+3 Fe3+2 [Si8O22] (OH)2

Sodic amphiboles are commonly blue, and often called blue amphibolesAmphibole Chemistry

Tremolite (Ca-Mg) occurs in meta-carbonatesActinolite occurs in low-grade metamorphosed basic igneous rocksOrthoamphiboles and cummingtonite-grunerite (all Ca-free, Mg-Fe-rich amphiboles) are metamorphic and occur in meta-ultrabasic rocks and some meta-sediments. The Fe-rich grunerite occurs in meta-ironstonesThe complex solid solution called hornblende occurs in a broad variety of both igenous and metamorphic rocksSodic amphiboles are predominantly metamorphic where they are characteristic of high P/T subduction-zone metamorphism (commonly called blueschist in reference to the predominant blue sodic amphiboles Riebeckite occurs commonly in sodic granitoid rocks

Amphibole Occurrences

InosilicatesPyroxenes and amphiboles are very similar:Both have chains of SiO4 tetrahedra The chains are connected into stylized I-beams by M octahedraHigh-Ca monoclinic forms have all the T-O-T offsets in the same directionLow-Ca orthorhombic forms have alternating (+) and (-) offsets+++++++++------+++aa++++++++++++------ClinopyroxeneOrthopyroxeneOrthoamphiboleClinoamphibole

InosilicatesCleavage 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 pyroxeneamphiboleab

SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5]Apical Os are unpolymerized and are bonded to other constituentsPhyllosilicates

Tetrahedral layers are bonded to octahedral layers (OH) pairs are located in center of T rings where no apical OPhyllosilicates

Octahedral layers can be understood by analogy with hydroxides

PhyllosilicatesBrucite: Mg(OH)2

Layers of octahedral Mg in coordination with (OH)

Large spacing along c due to weak van der waals bondsc

PhyllosilicatesGibbsite: Al(OH)3Layers of octahedral Al in coordination with (OH)Al3+ means that only 2/3 of the VI sites may be occupied for charge-balance reasonsBrucite-type layers may be called trioctahedral and gibbsite-type dioctahedrala1a2

PhyllosilicatesKaolinite: Al2 [Si2O5] (OH)4T-layers and diocathedral (Al3+) layers (OH) at center of T-rings and fill base of VI layer Yellow = (OH)T O - T O - T Ovdwvdwweak van der Waals bonds between T-O groups

PhyllosilicatesSerpentine: Mg3 [Si2O5] (OH)4T-layers and triocathedral (Mg2+) layers (OH) at center of T-rings and fill base of VI layer Yellow = (OH)T O - T O - T Ovdwvdwweak van der Waals bonds between T-O groups

SerpentineAntigorite maintains a sheet-like form by alternating segments of opposite curvatureChrysotile does not do this and tends to roll into tubes

SerpentineThe rolled tubes in chrysotile resolves the apparent paradox of asbestosform sheet silicatesS = serpentine T = talcNagby and Faust (1956) Am. Mineralogist 41, 817-836.Veblen and Busek, 1979, Science 206, 1398-1400.

PhyllosilicatesPyrophyllite: Al2 [Si4O10] (OH)2T-layer - diocathedral (Al3+) layer - T-layer T O T - T O T - T O Tvdwvdwweak van der Waals bonds between T - O - T groups Yellow = (OH)

PhyllosilicatesTalc: Mg3 [Si4O10] (OH)2T-layer - triocathedral (Mg2+) layer - T-layer T O T - T O T - T O Tvdwvdwweak van der Waals bonds between T - O - T groups Yellow = (OH)

PhyllosilicatesMuscovite: K Al2 [Si3AlO10] (OH)2 (coupled K - AlIV)T-layer - diocathedral (Al3+) layer - T-layer - KT O T K T O T K T O TK between T - O - T groups is stronger than vdw

PhyllosilicatesPhlogopite: K Mg3 [Si3AlO10] (OH)2T-layer - triocathedral (Mg2+) layer - T-layer - KT O T K T O T K T O TK between T - O - T groups is stronger than vdw

A Summary of Phyllosilicate StructuresPhyllosilicatesFig 13.84 Klein and Hurlbut Manual of Mineralogy, John Wiley & Sons

Chlorite: (Mg, Fe)3 [(Si, Al)4O10] (OH)2 (Mg, Fe)3 (OH)6= T - O - T - (brucite) - T - O - T - (brucite) - T - O - T -

Very hydrated (OH)8, so low-temperature stability (low-T metamorphism and alteration of mafics as cool)Phyllosilicates

Why are there single-chain-, double-chain-, and sheet-polymer types, and not triple chains, quadruple chains, etc??

Biopyriboles

It turns out that there are some intermediate types, predicted by J.B. Thompson and discovered in 1977 Veblen, Buseck, and Burnham

Cover of Science: anthophyllite (yellow) reacted to form chesterite (blue & green) and jimthompsonite (red)

Streaked areas are highly disorderedBiopyribolesCover of Science, October 28, 1977 AAAS

HRTEM image of anthophyllite (left) with typical double-chain widthJimthompsonite (center) has triple-chainsChesterite is an ordered alternation of double- and triple-chainsanthophyllitejimthompsonitechesteriteFig. 6, Veblen et al (1977) Science 198 AAAS

Disordered structures show 4-chain widths and even a 7-chain widthObscures the distinction between pyroxenes, amphiboles, and micas (hence the term biopyriboles: biotite-pyroxene-amphibole)BiopyribolesFig. 7, Veblen et al (1977) Science 198 AAAS

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

TectosilicatesLow Quartz001 Projection Crystal Class 32

TectosilicatesHigh Quartz at 581oC001 Projection Crystal Class 622

TectosilicatesCristobalite001 Projection Cubic Structure

TectosilicatesStishoviteHigh pressure SiVI

TectosilicatesLow Quartz StishoviteSiIV SiVI

TectosilicatesFeldsparsAlbite: NaAlSi3O8Substitute two Al3+ for Si4+ allows Ca2+ to be addedSubstitute Al3+ for Si4+ allows Na+ or K+ to be added