oligoclase

Oligoclase is a plagioclase feldspar mineral, a sodium-rich member of the plagioclase solid solution series between albite and andesine. Its composition typically includes about 10 to 30 percent anorthite (CaAl2Si2O8), the calcium-rich endmember, with the remainder being sodium aluminosilicate. In practice, oligoclase forms a continuous series with albite on the sodium-rich end and with andesine on the calcium-rich side.

Oligoclase occurs in a wide range of igneous and metamorphic rocks. It is common in granitic and granodioritic rocks, as well as in diorites and rhyolites, and can be found in high-grade metamorphic rocks such as gneisses and schists. Its presence often records fractionation during crystallization or particular metamorphic conditions. Oligoclase frequently occurs with other feldspars, including potassium feldspar, in perthitic intergrowths.

Crystallography and identification are characteristic of plagioclase feldspars. Oligoclase crystallizes in the triclinic system and shows two good cleavages at or near 90 degrees. It commonly displays albite-like twinning and distinct cleavage striations. It is typically colorless to white or pale gray and has a vitreous luster. Optical properties vary across the solid solution, making it distinguishable from more calcium- or sodium-rich members by composition-focused analysis rather than a single diagnostic property.

In use, oligoclase is not typically an ore mineral but is an important constituent of granitoids and felsic rocks, contributing to the mineralogical and petrological identity of the rocks in which it occurs.