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A Tour of the Rock Forming Silicates

A Tour of the Rock Forming Silicates. Common Silicates. 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

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A Tour of the Rock Forming Silicates

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  1. A Tour of the Rock Forming Silicates

  2. Common Silicates

  3. 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

  4. Structural Formulas • 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 • Olivine 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

  5. 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

  6. 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

  7. 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

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

  9. 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

  10. 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

  11. 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

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

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

  14. 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

  15. Inosilicates: Pyroxene Group • General pyroxene formula: (W,X,Y)2Z2O6

  16. 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 Inosilicates: Amphibole Group Ferro-actinolite

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

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

  19. 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

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

  21. 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 Phylosilicates: Other Serpentine Talc

  22. 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

  23. 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

  24. Plagioclase series: complete solid solution Tectosilicates: Feldspar Group • 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

  25. 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: Feldspar Group

  26. 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 Amethyst

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

  28. 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

  29. Minerals and Rocks

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