What happens to our PROTOLITH when acted on by AGENTS OF CHANGE Agents of Change T P fluids stress strain Metamorphic Reactions Solid solid phase transformation Solid solid net transfer Dehydration Hydration Decarbonation Carbonation Solid solid phase transformation Polymorphic reaction a mineral reacts to form a polymorph of that mineral No transfer of matter only a rearrangment of the mineral structure Example Andalusite Sillimanite Al2SiO5 Al2SiO5 Solid solid net transfer Involve solids only Differ from polymorphic transformations involve solids of differing composition and thus material must diffuse from one site to another for the reaction to proceed Examples NaAlSi2O6 SiO2 NaAlSi3O8 Jd Qtz Ab MgSiO3 CaAl2Si2O8 CaMgSi2O6 Al2SiO5 En An Di And Solid Solid Net Transfer II If minerals contain volatiles the volatiles must be conserved in the reaction so that no fluid phase is generated or consumed For example the reaction Mg3Si4O10 OH 2 4 MgSiO3 Mg7Si8O22 OH 2 Talc Enstatite Anthophyllite involves hydrous phases but conserves H2O It may therefore be treated as a solid solid net transfer reaction Hydration Dehydration Reactions Metamorphic reactions involving the expulsion or incorporation of water H 2O Example Al2Si4O10 OH 2 Al2SiO5 3SiO2 H2O Pyrophyllite And Ky Quartz water Carbonation Decarbonation Reactions Reactions that involve the evolution or consumption of CO2 CaCO3 SiO2 CaSiO3 CO2 calcite quartz wollastonite Reactions involving gas phases are also known as volatilization or devoltilization reactions These reactions can also occur with other gases such as CH4 methane H2 H2S O2 NH4 ammonia but they are not as common Systems Rock made of different minerals Metamorphic agents of change beat on it metamorphic reactions occur A closed system does not gain or lose material of any kind An open system can lose stuff liquids gases especially Outside world Hunk o rock Thermodynamics Primer Thermodynamics describes IF a reaction CAN occur at some condition T P composition typically Second Law of thermodynamics G H T S Where G Gibb s free energy determines IF the REACTION will go forward G spontaneous H is enthalpy has to do with heat S is entropy has to do with bonds and order Thermodynamics vs Kinetics Thermodynamics comparing the potential ENERGY of things what is more stable Will a reaction occur at some T P soln melt composition go or Not Kinetics IF thermodynamics says YES the reaction should occur always toward lower energy kinetics determines how fast Minerals out of equilibrium pass the thermodynamic test but the kinetics of their reaction is very slow Phase diagrams Tool for seeing phase transitions H2Oice H2Oliquid Reaction line governed by G H T S Phase Rule P F C 2 Phases coexisting degrees of freedom number of components 2 Degree of freedom 2 either axis can change and the phase stays the same where Phase diagrams Let s think about what happens to water as conditions change P F C 2 A C Point A Point B Point C B Mineral Assemblages in Metamorphic Rocks Equilibrium Mineral Assemblages At equilibrium the mineralogy and the composition of each mineral is determined by T P and X Relict minerals or later alteration products are thereby excluded from consideration unless specifically stated The Phase Rule in Metamorphic Systems Phase rule as applied to systems at equilibrium F C P 2 the phase rule P is the number of phases in the system C is the number of components the minimum number of chemical constituents required to specify every phase in the system F is the number of degrees of freedom the number of independently variable intensive parameters of state such as temperature pressure the composition of each phase etc 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 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 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 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 Prehnite Pumpellyite Minerals related