Orthoclase

Orthoclase, or orthoclase feldspar (endmember formula KAlSi₃O₈), is an important tectosilicatemineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture", because its two cleavage planes are at right angles to each other. It is a type of alkali feldspar, also known as potassium feldspar or K-spar. The gem known as moonstone (see below) is largely composed of orthoclase.