Augite
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Augite, also known as Augurite, is a common rock-forming pyroxene mineral with formula (Si, Al)₂O₆. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees. Wikipedia
Cleavage: good with 87° between, and; parting on, and
Streak: Greenish-white
Hardness: 5.5 to 6
Birefringence: δ = 0.026–0.039
Category: Inosilicates
Color: Black, brown, greenish, violet-brown; in thin section, colorless to gray with zoning common
Crystal habit: Commonly as stubby prismatic crystals, also acicular, skeletal, dendritic