Oligoclase

Oligoclase is a rock-forming mineral belonging to the plagioclasefeldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite (NaAlSi₃O₈) and anorthite (CaAl₂Si₂O₈). The albite:anorthite molar ratio of oligoclase 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.

Oligoclase is a common mineral in the more silica-rich varieties of igneous rock and in many metamorphic rocks.