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The Minerals of Metamorphosed Mafic Rocks

The 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) 4 O 8 (like in igneous rocks) Amphiboles (double-chain silicates)

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The Minerals of Metamorphosed Mafic Rocks

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  1. The Minerals of Metamorphosed Mafic Rocks Mafic rocks generally have igneous protoliths: basalt and its coarse-grained equivalent, gabbro

  2. Minerals of Metamorphosed Mafic Rocks • Plagioclase (Ca,Na)(Al,Si)4O8 • (like in igneous rocks) • Amphiboles (double-chain silicates) • Tremolite-Actinolite • Ca2(Mg,Fe)5Si8O22(OH)2 • light to dark green, colorless in thin section • Hornblende (complex formula related to trem.-actin.) • dark green to black, green or brown in thin section • Glaucophane (blue sodic amphibole)

  3. Actinolite

  4. Hornblende in thin section

  5. Glaucophane (blue amphibole)

  6. 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) • Orthopyroxene (enstatite) (Mg,Fe)2Si2O6 , or (Mg,Fe)SiO3 tan to brown parallel extinction (in microscope)

  7. More Minerals of Metamorphosed Mafic Rocks • Epidote (paired tetrahedra and isolated tetrahedra in a silicate) - view epidote movie • Ca2(Al,Fe3+)Al2O(SiO4)(Si2O7)(OH) • pistachio green (like canned peas) 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)

  8. View Epidote Structure Movie

  9. 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

  10. Metamorphic Facies in P and T Minerals in Meta-basalt • Greenschist - albite, actinolite, epidote, chlorite • Epidote Amphibolite - albite, hornblende, epidote • Amphibolite -plagioclase, hornblende, garnet

  11. Metamorphic Facies in P and T Minerals in Meta-basalt • Granulite - clino-pyroxene, ortho-pyroxene, plagioclase • Blueschist - albite, blue amphibole, lawsonite (paired sil.) • Eclogite - bright-green Na-rich pyroxene, garnet • More next semester!

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