Orthoclase, or
orthoclase feldspar (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. It is a type of
potassium feldspar, also known as
K-feldspar. The gem known as
moonstone (see below) is largely composed of orthoclase.