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

Silicates are classified on the basis of Si-O polymerism Mineral Structures Silicates are classified on the basis of Si-O polymerism [SiO4]4- Independent tetrahedra Nesosilicates Examples: olivine garnet [Si2O7]6- Double tetrahedra Sorosilicates Examples: lawsonite epidote n[SiO3]2- n = 3, 4, 6 Cyclosilicates Examples: benitoite BaTi[Si3O9] beryl Be3Al2[Si6O18]

Mineral Structures Inosilicates [SiO3]2- single chains Inosilicates [Si4O11]4- Double chains pryoxenes pyroxenoids amphiboles

Mineral Structures Phyllosilicates [Si2O5]2- Sheets of tetrahedra Phyllosilicates micas talc clay minerals serpentine

Mineral Structures Tectosilcates low-quartz [SiO2] 3-D frameworks of tetrahedra: fully polymerized Tectosilicates quartz and the silica minerals feldspars feldspathoids zeolites

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

Nesosilicates: Olivine (Mg,Fe)2SiO4 Olivine Occurrences: Principally in mafic and ultramafic igneous rocks- Typically ~60+% of mantle source for basalts- Fayalite in meta-ironstones and in some alkalic granitoids Forsterite in some siliceous dolomitic marbles

Nesosilicates: Garnet 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

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)

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

Inosilicates: single chains- pyroxenes The tetrahedral chain above the M1s is offset from that below The result is a monoclinic unit cell, hence clinopyroxenes e.g. Diopside, Augite (+) M2 c a (+) M1 (+) M2

an orthorhombic unit cell Inosilicates: single chains- pyroxenes Orthopyroxene an orthorhombic unit cell Enstatite (Mg2Si2O6) 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 Ca / (Ca + Na) Ca-Tschermack’s molecule 0.2 CaAl2SiO6 Augite Diopside-Hedenbergite Ca(Mg,Fe)Si2O6

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)

Inosilicates: double chains- amphiboles 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

Amphibole Chemistry 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

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 The complex solid solution called hornblende occurs in a broad variety of both igneous 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

Inosilicates pyroxene amphibole Cleavage angles can be interpreted in terms of weak bonds in M2 sites Narrow single-chain I-beams  90o cleavages in pyroxenes while wider double-chain I-beams  60-120o 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 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 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

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)

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

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