Royal Ontario Museum Polarised-Light Microscopy Laboratory

Tremolite - Actinolite

(AMPHIBOLES) Ca2(Mg,Fe)5[Si8O22](OH,F)2

Crystal system: Monoclinic
Refractive index: 1.599-1.705
Birefringence: 0.027-0.017
Cleavage: two good, 56o
Twinning: simple, lamellar common
Colour in PPL: colourless, pale green, dark green
Pleochroism: present in Fe-rich members
Relief: moderate
Distinction: lamellar twinning not as common nor as fine as in Cummingtonite, more Fe-rich members transitional to hornblende, distinguished by extinction angle between 10 and 15
Occurrence: regional and contact metamorphic rocks

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