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Mineral Structures

Mineral Structures. the [SiO 4 ] 4- tetrahedron. Silicates are classified on the basis of Si-O polymerism. Mineral Structures. [SiO 4 ] 4- Independent tetrahedra Nesosilicates Examples: olivine garnet [Si 2 O 7 ] 6- Double tetrahedra Sorosilicates

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Mineral Structures

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

  2. Mineral Structures [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] Silicates are classified on the basis of Si-O polymerism

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

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

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

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

  7. 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

  8. 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 Nesosilicates: Garnet Garnet (001) view blue = Si purple = A turquoise = B

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

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

  11. 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

  12. Inosilicates: single chains- pyroxenes Orthopyroxene an orthorhombic unit cell Enstatite (Mg2Si2O6) c (-) M1 (+) M2 a (+) M1 (-) M2

  13. 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

  14. 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 Ferrosilite Enstatite

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

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

  17. b Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 a sin Inosilicates: double chains- amphiboles 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

  18. 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

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

  20. 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”

  21. 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

  22. pyroxene amphibole Inosilicates b a 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

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

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

  25. 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

  26. 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

  27. 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

  28. 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)

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

  30. Tectosilicates Low Quartz Stishovite SiIVSiVI

  31. 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

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