feldspar

Close-up of a feldspar mineral. Feldspars are a group of rock-forming minerals that constitute almost 60% of the earth's crust.

Any of a group of silicate minerals. Feldspars are the most abundant mineral type in the Earth's crust. They are the chief constituents of igneous rock and are present in most metamorphic and sedimentary rocks. All feldspars contain silicon, aluminium, and oxygen, linked together to form a framework. Spaces within this framework structure are occupied by sodium, potassium, calcium, or occasionally barium, in various proportions. Feldspars form white, grey, or pink crystals and rank 6 on the Mohs scale of hardness.

The four extreme compositions of feldspar are represented by the minerals orthoclase, KAlSi₃O₈; albite, NaAlSi₃O₈; anorthite, CaAl₂Si₂O₈; and celsian, BaAl₂Si₂O₈. Plagioclase feldspars contain variable amounts of sodium (as in albite) and calcium (as in anorthite) with a negligible potassium content. Alkali feldspars (including orthoclase) have a high potassium content, less sodium, and little calcium.

The type known as moonstone has a pearl-like effect and is used in jewellery. Approximately 4,000 tonnes of feldspar are used in the ceramics industry annually.