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) 4 O 8 –(like in igneous rocks) Amphiboles(double-chain silicates) –Tremolite-Actinolite –Ca 2 (Mg,Fe) 5 Si 8 O 22 (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)Si 2 O 6 light to dark green inclined extinction (in microscope) –Orthopyroxene (enstatite) (Mg,Fe) 2 Si 2 O 6, or (Mg,Fe) SiO 3 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 –Ca 2 (Al,Fe 3+ )Al 2 O(SiO 4 )(Si 2 O 7 )(OH) –pistachio green (like canned peas) color is characteristic Chlorite (dark green sheet silicate) –(Mg,Fe,Al) 6 (Si,Al) 4 O 10 (OH) 8 Garnet (red, similar to that in meta-pelitic rocks)

8. View Epidote Structure Movie Epidote

9. 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

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

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