| Definitions | | oligoclase |
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noun
- (mineralogy) 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, <math>Na_xCa_xAlSi_3O_8</math>
- 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.
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