- 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, NaCaAlSiO ,Physical properties: Mohs 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.