A Tour of the Rock Forming Silicates

Common Silicates Group Igneous Metamorphic Sedimentary Neso (SiO4) Olivine group, Zircon, Topaz Garnet group, Alumnosilicates Soro (Si2O7) Epidote Epidote, Zoesite Cyclo (SiO3) Tourmaline Ino (single – SiO3 to Si2O6) Pyroxene group (Mg, Fe, Ca) Pyroxene group (Ca, Na) Ino (double – Si4O11) Amphibole (hornblende) Amphibole Phyllo (Si4O10) Mica (biotite-muscovite) Mica, Chlorite, Serpentine, Talc Clays Tecto (SiO2) Quartz, Feldspar Quartz, Feldspar, Zeolite

Structural Formulas and Silicates The key to understanding silicate mineral groups, solid solution, and miscibility Symbology W = large cations, C.N. >6 (with oxygen) Ca, Na, K X = medium-sized, bivalent cations, C.N. = 6 (with oxygen) Mg, Fe+2 , & Ca (sort of) Y = medium-sized, trivalent cation, C.N. = 6 (with oxygen) Typically Al and sometimes Fe+3 Z = small cations, C.N. = 4 (with oxygen) Mainly Si+4, but also Al+3

Structural Formulas Olivine Group Pyroxene Group Feldspar Group Nesosilicates [SiO4] X2SiO4 X = Mg, Fe Pyroxene Group Single chain inosilicates [SiO3 or Si2O6] X2SiO3 to (W,X,Y)2Z2O6 Feldspar Group Tecto (framework) silicates [SiO2] WZ4O8 Garnet Group Nesosilicates [SiO4] X3Y2 (SiO4)3 X = Ca, Mn, Fe, Mg Y = Fe+3, Cr Amphibole Group Double chain inosilicates [Si8O22] W0-1X2Y5(Z8O22)(OH, F)2 Mica Group Phylo (sheet) silicates [Si4O10] W(X,Y)2-3(Z4O10)(OH, F)2

Nesosilicates: Olivine Group Common component in ultramafic-mafic igneous rocks (peridotite, gabbro, basalt) A complete solid solution series X2SiO4 X = Mg, Fe 2:1 metals:Si Peridot (Forsterite) Dunite inclusion in basalt Dunite xenolith Flattened crystals of Fayalite

Nesosilicates: Olivine Group Complete solid solution: Forsterite: Mg2SiO4 Fayalite: Fe2SiO4 Also CaMg and CaFe end members, very rare Mg end member (Fo) crystallizes first Mg slightly more compatible than Fe Crystallize at high T

Nesosilicates: Garnet Group Garnet Group (X3Y2 {SiO4}3) X{A} –> divalent cations: Ca+2, Mn+2, Fe+2, Mg+2 Y{B} –> trivalent cations: Al+3, Fe+3, Cr+3 Generally high-grade (high temp and/or pressure) metamorphic rock occurrence Gem stone of high hardness: 7-7.5

Nesosilicates: Garnet Group Isomorphous minerals with some solid solution Pyralspites (Mg, Mn, Fe) Pyrope Almandine Spessartine Ugrandites (Ca) Uvarovite Grossular Andradite uvarovite

Nesosilicates: Other Accessory minerals in rocks Zircon (ZrSiO4) Commonly contains uranium and thorium (and daughter product: lead) as minor atomic substitution components Highly useful for geochronology: radiometric dating using the unstable isotopes U & Th -> Pb Also common as an accessory mineral in metamorphic and sedimentary (highly resistant) rocks Topaz (Al2SiO4 {OH,F}2) Pegmatite/hydrothermal accessory mineral Hard  a gem mineral

Sorosilicates and Cyclosilicates Sorosilicate: Epidote Group Common in regional metamorphism (epidote-ampibolite facies) Isostructural group with some solid solution Cyclosilicate: Tourmaline Common component in pegmatite Hosts incompatible elements Used as gemstone Tourmaline (“watermelon”) Epidote

Inosilicates: Pyroxene Group Single Chain Inosilicate Common in mafic igneous & metamorphic rocks Typically: X2Si2O6 X is the divalent metal cations: Mg, Fe, (Ca) Enstitite (Mg), Ferrosilite (Fe), Diopside (CaMg), Hedenburgite (CaFe) 1:1 metals:Si Some amount of Ca is also possible in the Pyroxene structure Wollastonite (Ca): not a pyroxene, rather a related mineral called a pyroxenoid

Inosilicates: Pyroxene Group The Pyroxene Quadrilateral “Real” pyroxenes Clinopyroxenes Diopside Hedenburgite AUGITE Orthopyroxenes Enstitite Ferrosilite HYPERSTHENE

Inosilicates: Pyroxene Group Pyroxene quadrilateral and miscibility gaps Gap between ortho- and clinopyroxene Accommodation of Ca Gap varies by temperature Ca deficient clinopyroxene: Augite Ca rich orthopyroxene: Pigeonite (hi-T only)

Inosilicates: Pyroxene Group Coupled Substitution Divalent/trivalent substitutions in tetrahedral (CN 4) and octahedral (CN 6) sites of many rock forming silicate minerals Substitution of cations with different valences Ex: Na+ for Ca2+ Maintaining overall charge balance requires additional substitutions Ex Na+ and Si4+ for Ca2+ and Al3+ In pyroxenes: Sodic pyrobole species Coupled substitution of Na+Al3+ for 2Ca2+ Jadeite->NaAlSi2O6 High pressure metamorphic minerals

Inosilicates: Pyroxene Group General pyroxene formula: (W,X,Y)2Z2O6 Chemistry Clinopyroxene CaMg Diopside (Cpx) Ca(MgFe) Augite (Cpx) CaFe Hedenburgite (Cpx ) Some Sodium Rich Varieties: NaFe Aegirine NaAl Jadeite (the gem) LiAl Spodumene

Inosilicates: Amphibole Group Double chain, hydrous silicates W0-1X2Y5(Z8O22)(OH)2 W = Na, K X = Ca, Na, Mn, Fe, Mg, Li Y = Mn, Fe, Mg, Fe3+, Al, Ti Z = Al, Si Closely related to pyroxenes Same cations; amphiboles have water Complete and partial solid solution Coupled substitution Orthorhombic and monoclinic Ferro-actinolite

Inosilicates: Amphibole Group Solid solution relationships and miscibility gap analogous to pyroxenes Mg-Fe complete solution series CaMg-CaFe solution series Miscibility gap due to accommodation of Ca NaAl coupled substitution series Ferro-actinolite

Inosilicates: Amphibole Group Hornblende: the most common (and a complicated) amphibole “Any black amphibole” Typical in intermediate igneous rocks Also common high temperature metamorphic rocks Hornblende (K,Na)0-1(Ca,Na,Fe,Mg)2 (Mg,Fe,Al)5(Si,Al)8O22(OH)2

Phylosilicates: Mica Group Hydrous sheet silicate Natural micas K(Al,Mg,Fe)2-3(AlSi3O10)(OH)2 Muscovite (Al micas) Biotite (Fe-Mg micas) Miscibility gap between biotite series and muscovite

Phylosilicates: Mica Group Biotite Essential minerals in Igneous rocks Muscovite: Felsic igneous rocks, Granites Biotite: Felsic to intermediate rocks Metamorphic rocks Schists pseudo-hexagonal crystalline aggregate of muscovite

Phylosilicates: Other Serpentine Serpentine Mg3Si2O5(OH)4 Low-grade alteration of olivine, pyroxene, and amphibole Talc Mg3Si4O10(OH)2 Low-grade metamorphic rocks Chlorite (Mg,Fe)3(Si,Al)4O10(OH)2 * (Mg,Fe)3(OH)6 Greenschist facies metamorphic rocks Talc

Tectosilicates: Feldspar Group The most abundant minerals in the Earth’s crust Framework silicates (SiO2) WZ4O8 (Ca,Na,K)(Al,Si)4O8 Plagioclase Series Anorthite to albite (Ca,Na)(Al,Si)4O8 Alkali Feldspars Orthoclase to albite (Na,K)(Al,Si)4O8

Tectosilicates: Feldspar Group Plagioclase Series Essential minerals in most igneous, sedimentary, and metamorphic rocks Complete (temperature dependant) solid solution between Albite (NaAlSi3O8) Anorthite (CaAl2Si2O8) Minor solid solution of K+ increasing with increasing Ab content

Tectosilicates: Feldspar Group Plagioclase series: complete solid solution Because of the similarity in ionic radius between Na+1 (0.95 Å) and Ca+2 (0.99 Å) Isomorphous solid solution between end members Coupled substitution (CaxNa1-x)(Al1+xSi3-x)O8 Ex: Ca.20Na.80(Al1.2Si2.8)O8 Oligoclase

Tectosilicates: Feldspar Group Alkali Feldspar: Miscibility gap Because of dissimilarity in size between the Na+1 (0.95 Å) and K+1 (1.33 Å) complete solid solution occurs only at high temp Wrong size “bricks” results in alkali feldspar polymorphs Single alkali feldspars formed at high temp exsolve (unmix) at lower temp (if slowly cooled) Orthoclase-rich-> perthite Albite-rich-> antiperthite

Tectosilicates: Quartz Group Amethyst Essentially “pure” SiO2 Component of many felsic and intermediate igneous rocks Not present in: Ultramafic igneous rocks Alkaline (feldspathoidal) igneous rocks Common particulate residue during bedrock weathering Common chemical precipitate in surface through hydrothermal settings Common component of metamorphic rocks

Tectosilicates: Quartz Group Silica P-T phase diagram Silica polymorphs in P-T “space” Hi P: coesite, stishovite Hi T: tridymite, cristobalite

Tectosilicates: Quartz Group Chalcedony: a micro- (very small) to crypto- crystalline (almost amorphous {non-crystalline}) fibrous quartz Common precipitate in surface and near-surface conditions

Minerals and Rocks Formation Essential minerals Silicate Non-silicate Igneous Crystallize from magma Olivine, pyroxene, amphibole, mica, feldspar, quartz -- Sedimentary Weathering residue Chemical precipitate Quartz, feldspar, clay Quartz (microcrystalline) -- Carbonates Halides Metamorphic Alteration of pre-existing mineral Garnet, epidote, pyroxene, amphibole, mica, quartz, feldspar Carbonates