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

Tectosilicates Stishovite High pressure  Si VI

Tectosilicates Low Quartz Stishovite Si IV 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

SiO 4 tetrahedra polymerized into 2-D sheets: [Si 2 O 5 ] 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 Phyllosilicates Brucite: Mg(OH) 2 Layers of octahedral Mg in coordination with (OH) Large spacing along c due to weak van der waals bonds c

Phyllosilicates Gibbsite: Al(OH) 3 Layers of octahedral Al in coordination with (OH) Al 3+ 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 a1a1a1a1 a2a2a2a2

Phyllosilicates Muscovite: K Al 2 [Si 3 AlO 10 ] (OH) 2 (coupled K - Al IV ) T-layer - diocathedral (Al 3+ ) layer - T-layer - K TOTKTOTKTOTTOTKTOTKTOTTOTKTOTKTOTTOTKTOTKTOT K between T - O - T groups is stronger than vdw

Phyllosilicates Phlogopite: K Mg 3 [Si 3 AlO 10 ] (OH) 2 T-layer - triocathedral (Mg 2+ ) layer - T-layer - K TOTKTOTKTOTTOTKTOTKTOTTOTKTOTKTOTTOTKTOTKTOT K between T - O - T groups is stronger than vdw

Aluminosilicate Minerals AndalusiteKyaniteSillimanite 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). 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 Al 2 SiO 5 are important metamorphic minerals AndalusiteKyaniteSillimanite

Serpentine Minerals Mg 3 Si 2 O 5 (OH) 4 minerals (principally as antigorite, lizardite, chrysotile polymorphs) Forms from hydration reaction of magnesium silicates –Mg 2 SiO H 2 O  Mg 3 Si 2 O 5 (OH) 4 + Mg(OH) 2 forsteriteserpentinebrucite Asbestosform variety is chrysotile (accounts for 95% of world’s asbestos production  MUCH LESS DANGEROUS than crocidolite)

Phyllosilicates Serpentine: Mg 3 [Si 2 O 5 ] (OH) 4 T-layers and triocathedral (Mg 2+ ) layers (OH) at center of T-rings and fill base of VI layer  Yellow = (OH) TO-TO-TOTO-TO-TOTO-TO-TOTO-TO-TO vdw vdw weak van der Waals bonds between T-O groups

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

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 (Mg 3 Mg 2 Al 2 Si 3 O 10 (OH) 8, chamosite (Fe), pennantite (Mn), nimmite (Ni) – end members Chloritoid (Fe,Mg,Mn) 2 Al 4 Si 2 O 10 (OH) 4 - Similar in appearance to chlorite, but different 2V and relief

Talc Hydrated magnesium silicate (Mg 3 Si 4 O 10 (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 (Ca 2 Al 2 Si 3 O 10 (OH) 2 is also green, pumpellyite (Ca 2 (Fe,Mg)Al 2 Si 3 O 11 (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 H 2 O –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 Ca 2 FeAl 2 Si 3 O 12 (OH), Zoisite (Ca 2 Al 3 Si 3 O 12 (OH) and clinozoisite (polymorph)

Garnets Garnet (001) view blue = Si purple = A turquoise = B Garnet: A 2+ 3 B 3+ 2 [SiO 4 ] 3 “Pyralspites” - B = Al Pyrope: Mg 3 Al 2 [SiO 4 ] 3 Almandine: Fe 3 Al 2 [SiO 4 ] 3 Spessartine: Mn 3 Al 2 [SiO 4 ] 3 “Ugrandites” - A = Ca Uvarovite: Ca 3 Cr 2 [SiO 4 ] 3 Grossularite: Ca 3 Al 2 [SiO 4 ] 3 Andradite: Ca 3 Fe 2 [SiO 4 ] 3 Occurrence: Mostly metamorphic Some high-Al igneous Also in some mantle peridotites

Staurolite Aluminosilicate - Fe 2 Al 9 Si 4 O 22 (OH) 2 Similar structure to kyanite with tetrahedrally coordinated Fe 2+ easily replaced by Zn 2+ and Mg 2+ Medium-grade metamorphic mineral, typically forms around 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 induces different metamorphic mineral assemblages (governed by thermodynamics/ kinetics) These assemblages are lumped into metamorphic facies (or grades)

The Phase Rule in Metamorphic Systems Consider the following three scenarios: C = 1 (Al 2 SiO 5 ) s F = 1 common s F = 2 rare s F = 3 only at the specific P-T conditions of the invariant point (~ 0.37 GPa and 500 o C) Figure The P-T phase diagram for the system Al 2 SiO 5 calculated using the program TWQ (Berman, 1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Let’s put it all together…

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

From Hacker, B.R.,