Systems - University of Vermont

Systems - University of Vermont

The Phase Rule in Metamorphic Systems Consider the following three scenarios: 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) 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. Representing Mineral Reactions albite jadeite + quartz

From Hacker, B.R., Lets put it all together What if we had staurolite and andalusite together? What conditions would that indicate? 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) 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 6fold). 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 worlds 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 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 Metamorphic Facies Where do we find these regimes of PT off of the typical continental isotherms?? How is the environment that forms a blueschist

facies rock different from one forming a hornfels? Metamorphic Facies Table 25-1. The definitive mineral assemblages that characterize each facies (for mafic rocks). Lets put it all together Facies Series

Miyashiro (1961) initially proposed five facies series, most of them named for a specific representative type locality The series were: 1. Contact Facies Series (very low-P) 2. Buchan or Abukuma Facies Series (low-P regional) 3. Barrovian Facies Series (medium-P regional) 4. Sanbagawa Facies Series (high-P, moderate-T) 5. Franciscan Facies Series (high-P, low T)

Fig. 25-3. Temperaturepressure diagram showing the three major types of metamorphic facies series proposed by Miyashiro (1973, 1994). Winter (2001) An

Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Isograds Lines (on a map) or Surfaces (in the 3D world) marking the appearance or disappearance of the Index minerals in rocks of appropriate composition

e.g. the garnet-in isograd; the stauroliteout isograd Complicated by the fact that most of these minerals are solid solutions Isograds for a single shale unit in southern Vermont Which side reflects a higher grade, or higher

P/T environment?

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