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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 Alkali feldspar polymorphs Single alkali feldspar crystal formed at high temp exsolves 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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