Silicates are classified on the basis of Si-O polymerism

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

Mineral Structures Silicates are classified on the basis of Si-O polymerism [SiO4] Independent tetrahedra Nesosilicates Examples: olivine garnet [Si2O7] Double tetrahedra Sorosilicates Examples: lawsonite n[SiO3]2- n = 3, 4, Cyclosilicates Examples: benitoite BaTi[Si3O9] axinite Ca3Al2BO3[Si4O12]OH beryl Be3Al2[Si6O18]

Mineral Structures Silicates are classified on the basis of Si-O polymerism [SiO3] single chains Inosilicates [Si4O11] Double tetrahedra pryoxenes pyroxenoids amphiboles

Mineral Structures Silicates are classified on the basis of Si-O polymerism [Si2O5] Sheets of tetrahedra Phyllosilicates micas talc clay minerals serpentine

Mineral Structures Silicates are classified on the basis of Si-O polymerism low-quartz [SiO2] D frameworks of tetrahedra: fully polymerized Tectosilicates quartz and the silica minerals feldspars feldspathoids zeolites

Nesosilicates: independent SiO4 tetrahedra
Mineral Structures Nesosilicates: independent SiO4 tetrahedra

b c projection Olivine (100) view blue = M1 yellow = M2

b c perspective Olivine (100) view blue = M1 yellow = M2

b M1 in rows and share edges M2 form layers in a-c that share corners Some M2 and M1 share edges a Olivine (001) view blue = M1 yellow = M2

b c M1 and M2 as polyhedra Olivine (100) view blue = M1 yellow = M2

Nesosilicates: independent SiO4 tetrahedra Olivine Occurrences:
Principally in mafic and ultramafic igneous and meta-igneous rocks Fayalite in meta-ironstones and in some alkalic granitoids Forsterite in some siliceous dolomitic marbles Monticellite CaMgSiO4 Ca  M2 (larger ion, larger site) High grade metamorphic siliceous carbonates

Garnet: A2+3 B3+2 [SiO4]3 “Pyralspites” - B = Al Pyrope: Mg3 Al2 [SiO4]3 Almandine: Fe3 Al2 [SiO4]3 Spessartine: Mn3 Al2 [SiO4]3 “Ugrandites” - A = Ca Uvarovite: Ca3 Cr2 [SiO4]3 Grossularite: Ca3 Al2 [SiO4]3 Andradite: Ca3 Fe2 [SiO4]3 Occurrence: Mostly metamorphic Some high-Al igneous Also in some mantle peridotites Garnet (001) view blue = Si purple = A turquoise = B

Garnet: A2+3 B3+2 [SiO4]3 “Pyralspites” - B = Al Pyrope: Mg3 Al2 [SiO4]3 Almandine: Fe3 Al2 [SiO4]3 Spessartine: Mn3 Al2 [SiO4]3 “Ugrandites” - A = Ca Uvarovite: Ca3 Cr2 [SiO4]3 Grossularite: Ca3 Al2 [SiO4]3 Andradite: Ca3 Fe2 [SiO4]3 Occurrence: Mostly metamorphic Pyralspites in meta-shales Ugrandites in meta-carbonates Some high-Al igneous Also in some mantle peridotites a2 a1 a3 Garnet (001) view blue = Si purple = A turquoise = B

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

b a sin Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

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

SiO4 as polygons (and larger area) IV slab VI slab IV slab a sin VI slab IV slab VI slab IV slab b Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

M1 octahedron

(+) M1 octahedron (+) type by convention

M1 octahedron This is a (-) type (-)

M1 Creates an “I-beam” like unit in the structure.

(+) T M1 Creates an “I-beam” like unit in the structure

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

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

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 clinopyroxenes (+) M2 c a (+) M1 (+) 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) c (-) M1 (+) M2 a (+) M1 (-) M2

Pyroxene Chemistry The general pyroxene formula: W1-P (X,Y)1+P Z2O6
Where W = Ca Na X = Mg Fe2+ Mn Ni Li Y = Al Fe3+ Cr Ti Z = Si Al Anhydrous so high-temperature or dry conditions favor pyroxenes over amphiboles

Pyroxene Chemistry The pyroxene quadrilateral and opx-cpx solvus
Coexisting opx + cpx in many rocks (pigeonite only in volcanics) Wollastonite pigeonite 1200oC orthopyroxenes clinopyroxenes 1000oC Diopside Hedenbergite clinopyroxenes Solvus 800oC pigeonite (Mg,Fe)2Si2O6 Ca(Mg,Fe)Si2O6 orthopyroxenes Enstatite Ferrosilite

Ca-Tschermack’s molecule
Pyroxene Chemistry “Non-quad” pyroxenes Jadeite Aegirine NaAlSi2O6 NaFe3+Si2O6 0.8 Omphacite aegirine- augite Spodumene: LiAlSi2O6 Ca / (Ca + Na) Ca-Tschermack’s molecule 0.2 CaAl2SiO6 Augite Diopside-Hedenbergite Ca(Mg,Fe)Si2O6

Pyroxenoids “Ideal” pyroxene chains with 5.2 A repeat (2 tetrahedra) become distorted as other cations occupy VI sites 7.1 A 12.5 A 17.4 A 5.2 A Pyroxene 2-tet repeat Wollastonite (Ca  M1)  3-tet repeat Rhodonite MnSiO3  5-tet repeat Pyroxmangite (Mn, Fe)SiO3  7-tet repeat

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

Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 a sin 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 = H

Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 Same I-beam architecture, but the I-beams are fatter (double chains) 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)2 (+) (+) (+) Same I-beam architecture, but the I-beams are fatter (double chains) a sin (+) (+) All are (+) on clinoamphiboles and alternate in orthoamphiboles 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 = H

Hornblende: (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 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 = H

Hornblende: (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) 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 = H

Amphibole Chemistry See handout for more information General formula:
W0-1 X2 Y5 [Z8O22] (OH, F, Cl)2 W = Na K X = Ca Na Mg Fe2+ (Mn Li) Y = Mg Fe2+ Mn Al Fe3+ Ti Z = Si Al Again, the great variety of sites and sizes  a great chemical range, and hence a broad stability range The hydrous nature implies an upper temperature stability limit

Amphibole Chemistry Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes) Tremolite Ferroactinolite Ca2Mg5Si8O22(OH)2 Actinolite Ca2Fe5Si8O22(OH)2 Clinoamphiboles Cummingtonite-grunerite Anthophyllite Mg7Si8O22(OH)2 Fe7Si8O22(OH)2 Orthoamphiboles Al 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 compositions

Amphibole Chemistry 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)2 Riebeckite: Na2 Fe2+3 Fe3+2 [Si8O22] (OH)2 Sodic amphiboles are commonly blue, and often called “blue amphiboles”

Amphibole Occurrences
Tremolite (Ca-Mg) occurs in meta-carbonates Actinolite occurs in low-grade metamorphosed basic igneous rocks Orthoamphiboles 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-ironstones The complex solid solution called hornblende occurs in a broad variety of both igenous and metamorphic rocks Sodic 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

Inosilicates - - - - - - - - - - - -
+ + + + + + a + + + + + + + + + + + + - - - - - Clinopyroxene - Clinoamphibole + + a + + + + - - - - - - Orthopyroxene Orthoamphibole Pyroxenes and amphiboles are very similar: Both have chains of SiO4 tetrahedra The chains are connected into stylized I-beams by M octahedra High-Ca monoclinic forms have all the T-O-T offsets in the same direction Low-Ca orthorhombic forms have alternating (+) and (-) offsets

Inosilicates pyroxene amphibole
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  o cleavages in amphiboles

Phyllosilicates SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5]
Apical O’s are unpolymerized and are bonded to other constituents

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

Phyllosilicates Octahedral layers can be understood by analogy with hydroxides Brucite: Mg(OH)2 Layers of octahedral Mg in coordination with (OH) Large spacing along c due to weak van der waals bonds c

Phyllosilicates a2 a1 Gibbsite: Al(OH)3
Layers of octahedral Al in coordination with (OH) Al3+ means that only 2/3 of the VI sites may be occupied for charge-balance reasons Brucite-type layers may be called trioctahedral and gibbsite-type dioctahedral

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

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

Chrysotile does not do this and tends to roll into tubes
Serpentine 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

Serpentine Veblen and Busek, 1979, Science 206, S = serpentine T = talc Nagby and Faust (1956) Am. Mineralogist 41, The rolled tubes in chrysotile resolves the apparent paradox of asbestosform sheet silicates

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

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

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

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

Phyllosilicate Structures
A Summary of Phyllosilicate Structures Fig Klein and Hurlbut Manual of Mineralogy, © John Wiley & Sons

Phyllosilicates 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)

“Biopyriboles” 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 disordered Cover of Science, October 28, 1977 © AAAS

HRTEM image of anthophyllite (left) with typical double-chain width
Fig. 6, Veblen et al (1977) Science 198 © AAAS anthophyllite jimthompsonite chesterite HRTEM image of anthophyllite (left) with typical double-chain width Jimthompsonite (center) has triple-chains Chesterite is an ordered alternation of double- and triple-chains

“Biopyriboles” Fig. 7, Veblen et al (1977) Science 198 © AAAS Disordered structures show 4-chain widths and even a 7-chain width Obscures the distinction between pyroxenes, amphiboles, and micas (hence the term biopyriboles: biotite-pyroxene-amphibole)

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

Tectosilicates Low Quartz 001 Projection Crystal Class 32

Tectosilicates High Quartz at 581oC 001 Projection Crystal Class 622

Tectosilicates Cristobalite 001 Projection Cubic Structure

Tectosilicates Stishovite High pressure  SiVI

Tectosilicates Low Quartz Stishovite SiIV SiVI

Tectosilicates Feldspars
Substitute Al3+ for Si4+ allows Na+ or K+ to be added Substitute two Al3+ for Si4+ allows Ca2+ to be added Albite: NaAlSi3O8

Silicates are classified on the basis of Si-O polymerism

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Silicates are classified on the basis of Si-O polymerism

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