Pigeonite
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Pigeonite is a mineral in the clinopyroxene subgroup of the pyroxene group. It has a general formula of Si₂O₆. The calcium cation fraction can vary from 5% to 25%, with iron and magnesium making up the rest of the cations. Wikipedia
Optical properties: Biaxial (+)
Cleavage: Good on, (110) ^ (110) ~87°
2V angle: 0 – 30° measured
Birefringence: δ = 0.021 – 0.030
Crystal class: Prismatic (2/m); (same H-M symbol)
Crystal habit: Prismatic crystals, to 1 cm; granular, massive
Crystal system: Monoclinic