English

Noun

1. A plagioclase feldspar, the second member of the Albite-Anorthite solid solution series. Primarily found as small crystals in impure marble. Oligoclase contains a small amount of calcium substituting for some of the sodium in its formula. Oligoclase with reddish-golden inclusions found in Norway and Canada is called sunstone.

• Chemical composition: Sodium calcium aluminum silicate, Na_xCa_xAlSi₃O₈

• Physical properties:
  • Moh's Hardness: 6.0
  • Specific gravity: 2.65
  • Color: white, yellow, reddish-gray
  • Luster: glassy
  • Fracture: conchoidal
  • Cleavage: two good at about 94º to each other
  • Habit: Triclinic - pinacoidal. Usually twinned and striated, but this may not be visible to the unaided eye.