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Oligoclase

Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite and anorthite. The albite:anorthite molar ratio of... Wikipedia
Cleavage: Perfect, Good, Poor
Color: Usually white, with shades of grey, green, or red
Crystal class: Pinacoidal (1); (same H-M symbol)
Crystal system: Triclinic
Formula (repeating unit): (Ca,Na)(Al,Si)4O8, where Ca/(Ca + Na) (% anorthite) is between 10–30%
Fracture: Uneven, sub-conchoidal
Luster: Vitreous