Noun

1. 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 butterfly's wing.

• Chemical composition: Calcium sodium aluminum silicate, $Ca\_xNa\_xAlSi\_3O\_8$

• Physical properties:
  • Moh's 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.