What is "oligoclase"

Oligoclase \Ol"i*go*clase\, n. [Oligo- + Gr. ? fracture, fr. ? to break.] (Min.) A triclinic soda-lime feldspar. See Feldspar.

n. (context mineralogy English) 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''.

n. any of a rock-forming series of triclinic feldspars [syn: plagioclase]

Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite ( Na Al Si O) and anorthite ( CaAlSiO). The albite:anorthite molar ratio ranges from 90:10 to 70:30.

Oligoclase is a high sodium feldspar crystallizing in the triclinic system. The Mohs hardness is 6 to 6.5 and the specific gravity is 2.64 to 2.66. The refractive indices are: nα=1.533–1.543, nβ=1.537–1.548, and nγ=1.542–1.552. In color it is usually white, with shades of grey, green, or red.