Orthoclase

Orthoclase (endmember formula KAlSi₃O₈) is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar. The gem known as moonstone (see below) is largely composed of orthoclase.