alkali feldspar

Any of several common silicate minerals that often occur as whitish to variously coloured, glassy crystals. The alkali feldspars may be regarded as mixtures of sodium aluminosilicate (NaAlSiO) and potassium aluminosilicate (KAlSiO). Both the sodium and potassium aluminosilicates have several distinct forms, each with a different structure. Alkali feldspars are used in the manufacture of glass and ceramics; transparent, highly coloured, or iridescent varieties are sometimes used as gemstones. They are primarily important, however, as constituents of rocks (see plagioclase).