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Cummingtonite - Grunerite
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(AMPHIBOLES) (Mg,Fe)7[Si8O22)(OH)2
Crystal system: Monoclinic
Refractive index: 1.635-1.729
Birefringence: 0.020-0.045
Cleavage: two good, 55o
Twinning: simple, lamellar, very common
Colour in PPL: colourless to pale green
Pleochroism: Mg-rich none, Fe-rich colourless or pale brown to pale green
Relief: moderate to high
Distinction: higher releif than tremolite, distinctive narrow lamellar twinning
Occurrence: regional metamorphosed rocks
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