A member of the plagioclase feldspar group and the Ca-rich end member of the albite-anorthite series, this mineral is one of the rock-forming minerals. It forms in mafic plutonic rocks. Its name comes from the Greek word meaning "not right angle" or "oblique," in allusion to the oblique triclinic form of the crystals.

Mineral Photos courtesy of R.Weller/Cochise College