Orthoclase is a rock-forming feldspar mineral that is common in granites, syenites, and high grade metamorphic rocks. It is usually recognized by its color, hardness, and cleavage. It is distinguished from the other feldspars by its right-angle cleavage and the lack of twin striations on the best cleavage surface. The name comes from the Greek orthos meaning "right" and kalo meaning "I cleave," in allusion to its right angle of good cleavage.

Mineral Photos courtesy of R.Weller/Cochise College