A plagioclase feldspar, the fourth member of the Albite-Anorthite solid solution series. Often very coarsely crystalline, it is used as a decorative stone for carvings and building façades. Crystal surfaces often show a bluish reflection called "Schiller", which resembles the reflections from a butterflys wing. ,Chemical composition: Calcium sodium aluminum silicate, ,Physical properties: Mohs Hardness: 6.0 Specific gravity: 2.71 Color: Usually bluish-gray to dark gray Luster: glassy Fracture: conchoidal Cleavage: two good at about 94º to each other Habit: Triclinic - pinacoidal. Crystals are common and usually striated.
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