SiO2 Stishovite Pressure GPa 10 8 6 Coesite 4 2 quartz quartz 600 Liquid Cristobalite Tridymite 1000 1400 1800 2200 Temperature oC 2600 After Swamy and Saxena 1994 J Geophys Res 99 11 787 11 794 Tectosilicates Stishovite Coesite Low Quartz quartz quartz Cristobalite Tridymite 001 Projection Crystal Class 32 Liquid Tectosilicates Stishovite Coesite High Quartz at 581oC quartz quartz Cristobalite Tridymite 001 Projection Crystal Class 622 Liquid Tectosilicates Stishovite Coesite Cristobalite quartz quartz Cristobalite Tridymite 001 Projection Cubic Structure Liquid Tectosilicates Stishovite Coesite Stishovite quartz quartz Cristobalite Tridymite High pressure SiVI Liquid Tectosilicates Low Quartz SiIV Stishovite Si VI 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 Phyllosilicates SiO4 tetrahedra polymerized into 2 D sheets Si2O5 Apical O s are unpolymerized and are bonded to other constituents Phyllosilicates Tetrahedral layers are bonded to octahedral layers OH pairs are located in center of T rings where no apical O Phyllosilicates Octahedral layers can be understood by analogy with hydroxides Brucite Mg OH 2 c Layers of octahedral Mg in coordination with OH Large spacing along c due to weak van der waals bonds 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 Phyllosilicates 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 T O T K T O T K T O T Phyllosilicates Phlogopite K Mg3 Si3AlO10 OH 2 T layer triocathedral Mg2 layer T layer K K between T O T groups is stronger than vdw T O T K T O T K T O T 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 6fold 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 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 Yellow OH Serpentine Mg3 Si2O5 OH 4 T layers and triocathedral Mg2 layers OH at center of T rings and fill base of VI layer weak van der Waals bonds between T O groups T O T O T O vdw vdw Serpentine Antigorite maintains a sheet like form by alternating segments of opposite curvature Chrysotile does not do this and tends to roll into tubes Octahedra are a bit larger than tetrahedral match so they cause bending of the T O layers after Klein and Hurlbut 1999 Serpentine Nagby and Faust 1956 Am Mineralogist 41 817 836 Veblen and Busek 1979 Science 206 1398 1400 S serpentine T talc The rolled tubes in chrysotile resolves the apparent paradox of asbestosform sheet silicates 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 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 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 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 Metamorphic facies P T conditions presence of fluids