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.

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

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

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)

Phyllosilicates T O - T O - T O Serpentine: Mg3 [Si2O5] (OH)4 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

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

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

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

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

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

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

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

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

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

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