Oligoclase

Article By:

Griffen, Dana T. Department of Geology, Brigham Young University, Provo, Utah.

Last reviewed:December 2019

DOI:https://doi.org/10.1036/1097-8542.468200

A plagioclase feldspar with composition in the range Ab₉₀An₁₀ to Ab₇₀An₃₀, where Ab represents the composition of albite, NaAlSi₃O₈ and An represents the composition of anorthite, CaAl₂Si₂O₈. The diagnostic properties are hardness on Mohs scale, 6–6.5; density, 2.65 g/cm³; two good cleavages that intersect at approximately 90°; and color usually white or colorless (see illustration), transparent to translucent. The presence of minute, mutually parallel inclusions of hematite (Fe₂O₃) causes a golden play of color in the variety of oligoclase called aventurine or sunstone. Repeated twinning is common and results in closely spaced striations visible with a hand-held magnifier. Mean refractive index is 1.545; the mineral may be optically positive or negative, depending on composition. Oligoclase is triclinic. The mineral is common in both plutonic and volcanic silicic igneous rocks, as well as in quartzofeldspathic and pelitic metamorphic rocks. See also: Albite; Anorthite; Crystal structure

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