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SiO 2

SiO 2. After Swamy and Saxena (1994) J. Geophys. Res., 99 , 11,787-11,794. Tectosilicates. Low Quartz. 001 Projection Crystal Class 32. Tectosilicates. High Quartz at 581 o C. 001 Projection Crystal Class 622. Tectosilicates. Cristobalite. 001 Projection Cubic Structure.

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SiO 2

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  1. SiO2 After Swamy and Saxena (1994)J. Geophys. Res., 99, 11,787-11,794.

  2. Tectosilicates Low Quartz 001 Projection Crystal Class 32

  3. Tectosilicates High Quartz at 581oC 001 Projection Crystal Class 622

  4. Tectosilicates Cristobalite 001 Projection Cubic Structure

  5. Tectosilicates Stishovite High pressure  SiVI

  6. Tectosilicates Low Quartz Stishovite SiIVSiVI

  7. Micas • Biotite and Muscovite are also important metamorphic minerals (muscovite often the principle component of schists) • Phlogopite – similar to biotite, but has little iron, forms from Mg-rich carbonate deposits and a common mineral in kimberlites (diamond-bearing material) • Sericite – white mica (similar to muscovite) – common product of plagioclase feldspar alteration at low grades

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

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

  10. Phyllosilicates Octahedral layers can be understood by analogy with hydroxides Brucite: Mg(OH)2 Layers of octahedral Mg in coordination with (OH) Large spacing along c due to weak van der waals bonds c

  11. 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

  12. 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

  13. 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

  14. Aluminosilicate Minerals • SILLIMANITE: Orthorhombic: Octahedral Al chains (6-fold) are crosslinked by both Si and Al tetrahedra (4-fold). • ANDALUSITE: Orthorhombic: 5-coordinated Al; Same octahedral (6-fold) chains. • KYANITE: Triclinic: All the Al is octahedrally coordinated (6- and 6-fold). Andalusite Kyanite Sillimanite • Clearly, changes in structure are in response to changing P and T. Result is changes in Al coordination. • Phase transformations require rebonding of Al. Reconstructive polymorphism requires more energy than do displacive transformations. Metastability of these 3 are therefore important (Kinetic factors limit equilibrium attainment). • All 3 are VERY important metamorphic index minerals.

  15. Aluminosilicate Minerals • 3 polymorphs of Al2SiO5 are important metamorphic minerals Andalusite Kyanite Sillimanite

  16. Serpentine Minerals • Mg3Si2O5(OH)4 minerals (principally as antigorite, lizardite, chrysotile polymorphs) • Forms from hydration reaction of magnesium silicates • Mg2SiO4 + 3 H2O  Mg3Si2O5(OH)4+ Mg(OH)2 forsterite serpentine brucite • Asbestosform variety is chrysotile (accounts for 95% of world’s asbestos production  MUCH LESS DANGEROUS than crocidolite)

  17. Phyllosilicates T O - T O - T O Yellow = (OH) vdw Serpentine: Mg3 [Si2O5] (OH)4 T-layers and triocathedral (Mg2+) layers (OH) at center of T-rings and fill base of VI layer  vdw weak van der Waals bonds between T-O groups

  18. Octahedra are a bit larger than tetrahedral match, so they cause bending of the T-O layers (after Klein and Hurlbut, 1999). Serpentine Antigorite maintains a sheet-like form by alternating segments of opposite curvature Chrysotile does not do this and tends to roll into tubes

  19. Serpentine Veblen and Busek, 1979, Science 206, 1398-1400. S = serpentine T = talc Nagby and Faust (1956) Am. Mineralogist 41, 817-836. The rolled tubes in chrysotile resolves the apparent paradox of asbestosform sheet silicates

  20. Chlorite Group • Another phyllosilicate, a group of difficult to distinguish minerals • Typically green, and the dominant and characteristic mineral of greenschist facies rocks • Forms from the alteration of Mg-Fe silicates (pyroxenes, amphiboles, biotite, garnets) • Clinochlore (Mg3Mg2Al2Si3O10(OH)8, chamosite (Fe), pennantite (Mn), nimmite (Ni) – end members • Chloritoid (Fe,Mg,Mn)2Al4Si2O10(OH)4 - Similar in appearance to chlorite, but different 2V and relief

  21. Talc • Hydrated magnesium silicate (Mg3Si4O10(OH)2) – metamorphic product of Mg-rich aluminosilicates (pyroxenes, amphiboles, olivines, serpentines, dolomite) Soapstone is a rock composed mostly of high-grade talc

  22. Prehnite-Pumpellyite • Low-grade metamorphic minerals • Minerals related to chlorite, form at slightly lower P-T conditions • Prehnite (Ca2Al2Si3O10(OH)2 is also green, pumpellyite (Ca2(Fe,Mg)Al2Si3O11(OH)2) is green too, varies based on Fe content • Prehnite + chlorite  pumpellyite + quartz

  23. Zeolites • Diverse group of minerals forming at lower metamorphic grades • Framework silicas, but characteristically containing large voids and highly variable amounts of H2O • Name is from the greek – meaning to boil stone as the water can de driven off with heat • Voids can acts as molecular sieves and traps for many molecules • Diversity of minerals in this group makes a for a wide variety of sieve and trapping properties selective for different molecules

  24. Epidote Group • Sorosilicates (paired silicate tetrahedra) • Include the mineral Epidote Ca2FeAl2Si3O12(OH), Zoisite (Ca2Al3Si3O12(OH) and clinozoisite (polymorph)

  25. 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 Garnets Garnet (001) view blue = Si purple = A turquoise = B

  26. Staurolite • Aluminosilicate - Fe2Al9Si4O22(OH)2 • Similar structure to kyanite with tetrahedrally coordinated Fe2+ easily replaced by Zn2+ and Mg2+ • Medium-grade metamorphic mineral, typically forms around 400-500 C • chloritoid + quartz = staurolite + garnet • chloritoid + chlorite + muscovite = staurolite + biotite + quartz + water • Degrades to almandine (garnet at higher T) • staurolite + muscovite + quartz = almandine + aluminosilicate + biotite + water

  27. Metamorphic chain silicates • Actinolite and tremolite are chain silicates derived from dolomite and quartz and common in low-mid grade metamorphic rocks • Riebeckite and Glaucophane are also chain silicates – higher grade minerals, often a blue color • These minerals usually lower P, higher T conditions

  28. Metamorphic facies • P-T conditions, presence of fluids induces different metamorphic mineral assemblages (governed by thermodynamics/ kinetics) • These assemblages are lumped into metamorphic facies (or grades)

  29. The Phase Rule in Metamorphic Systems C = 1 (Al2SiO5) • F = 1 common • F = 2 rare • F = 3 only at the specific P-T conditions of the invariant point (~ 0.37 GPa and 500oC) Consider the following three scenarios: Figure 21-9. The P-T phase diagram for the system Al2SiO5 calculated using the program TWQ (Berman, 1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  30. Let’s put it all together…

  31. What if we had staurolite and andalusite together? What conditions would that indicate?

  32. From Hacker, B.R.,

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