Geology 284 Mineralogy Fall 2008 Dr Helen Lang West Virginia University Minerals of Metamorphosed Mafic Rocks Mafic rocks generally have igneous protoliths basalt and its coarse grained equivalent gabbro Minerals of Metamorphosed Mafic Rocks Plagioclase Ca Na Al Si 4O8 like in igneous rocks Amphiboles double chain silicates Common properties of all amphiboles Tremolite Actinolite Ca2 Mg Fe 5Si8O22 OH 2 light to dark green colorless in thin section Hornblende complex formula related to tremolite actinolite dark green to black green or brown in thin section Glaucophane blue sodic amphibole Na2Mg3Al2 Si8O22 OH 2 Actinolite Hornblende in thin section Glaucophane blue amphibole More Minerals of Metamorphosed Mafic Rocks Pyroxenes similar to those in igneous rocks Clinopyroxene diopside Ca Mg Fe Si2O6 light to dark green inclined extinction in microscope moderate birefringence Orthopyroxene enstatite Mg Fe 2Si2O6 or Mg Fe SiO3 tan to brown parallel extinction in microscope low birefringence More Minerals of Metamorphosed Mafic Rocks Epidote paired tetrahedra and isolated tetrahedra in a silicate Ca2 Al Fe3 Al2O SiO4 Si2O7 OH pistachio green color is characteristic Chlorite dark green sheet silicate Mg Fe Al 6 Si Al 4O10 OH 8 Garnet red similar to that in meta pelitic rocks Ca Fe Mg Mn 3Al2Si3O12 View Epidote Structure Movie http socrates berkeley edu eps2 wisc geo360 epidote mov Metamorphic Facies Different combinations of these minerals occur at different metamorphic conditions In 1920 Pentii Eskola a geology professor at the University of Helsinki Finland introduced the idea of Metamorphic Facies Metamorphic Facies group minerals of rocks that experienced similar ranges of pressure and temperature during metamorphism Metamorphic Facies are named for minerals in metamorphosed mafic rocks or meta basalts Metamorphic Facies in P and T see handout Minerals in Moderate Pressure Metamorphic Facies you fill them in Greenschist Epidote Amphibolite Amphibolite Granulite Minerals in High Pressure Metamorphic Facies Blueschist Eclogite
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