Omphacite
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Omphacite is a member of the clinopyroxene group of silicate minerals with formula: Si₂O₆. It is a variably deep to pale green or nearly colorless variety of clinopyroxene. It normally appears in eclogite, which is the high-pressure metamorphic... Wikipedia
Optical properties: Biaxial (+)
Birefringence: δ = 0.023
2V angle: Measured: 58° to 83°, Calculated: 74° to 88°
Cleavage: Good on,^≈87°; parting on
Color: Green to dark green; colorless to pale green in thin section
Crystal class: Prismatic (2/m); (same H-M symbol)
Crystal habit: Rarely in rough crystals; anhedral, granular to massive