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Aluminosilicate Minerals

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).

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Aluminosilicate Minerals

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  1. 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.

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

  3. Topaz • Aluminosilicate mineral as well, one oxygen substituted with OH, F • Al2SiO4(F,OH)2 • Where do you think Topaz forms??

  4. 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)

  5. 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

  6. 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

  7. 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

  8. Chlorite • 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, chamosite, pennantite, nimmite – end members • Chloritoid - Similar in appearance to chlorite, but different 2V and relief

  9. Prehnite-Pumpellyite • Low-grade metamorphic minerals • Minerals related to chlorite, form at slightly lower P-T conditions • Prehnite is also green, pumpellyite green too, varies based on Fe content • Prehnite + chlorite  pumpellyite + quartz

  10. 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

  11. 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

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

  13. 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

  14. 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

  15. 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

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