Orthoclase

Orthoclase, or orthoclase feldspar (end-member formula KAlSi3O8), is an important tectosilicate mineral that forms igneous rock. Its name comes 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, and it is a common constituent of most granites and other felsic igneous rocks, often forming huge crystals and masses in pegmatite. The gem known as moonstone is largely composed of orthoclase. In this atlas it is the mineral species that moonstone records, kept apart from the broader feldspar entity.

Orthoclase, or orthoclase feldspar (end-member formula KAlSi3O8), is an important tectosilicate mineral that forms igneous rock. Its name comes 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, and it is a common constituent of most granites and other felsic igneous rocks, often forming huge crystals and masses in pegmatite. The gem known as moonstone is largely composed of orthoclase. In this atlas it is the mineral species that moonstone records, kept apart from the broader feldspar entity.