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