Any of several common silicate mineral s that often occur as whitish to variously coloured, glassy crystals.
The alkali feldspar s may be regarded as mixtures of sodium aluminosilicate (NaAlSi 3 O 8 ) and potassium aluminosilicate (KAlSi 3 O 8 ). 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 ).