Feldspar is any of a group of rock-forming, aluminosilicate (containing aluminum and silicon) minerals that also contain one or two of the following metals: calcium, sodium, potassium, and, rarely, barium. Potash feldspars, principally orthoclase and microcline, are potassium aluminum silicates where sodium or barium may partly replace potassium. Plagioclase feldspars form a series derived from sodium aluminum silicate (albite) with calcium replacing sodium in all proportions up to 100% (anorthite).
Feldspars make up about 60% of Earth's crust and are the major component in nearly all igneous rocks found on Earth, on the Moon, and in some meteorites. They also are common in metamorphic and some sedimentary rocks. Their complex chemical and structural properties make them useful for interpreting the origins of rocks.
Feldspars are industrially important in glass and ceramic industries, pottery and enamelware, soaps, abrasives, bond for abrasive wheels, cements and concretes, insulating compositions, fertilizer, poultry grit, tarred roofing materials, and as a sizing (or filler) in textiles and paper.