A sodium-rich plagioclase feldspar mineral whose composition extends over the range Ab₁₀₀An₀ to Ab₉₀An₁₀, where Ab (= albite) is NaAlSi₃O₈ and An (= anorthite) is CaAl₂Si₂O₈ (see illus.). Albite occurs in crustal igneous rocks as a major component of pegmatites and granites, in association with quartz, mica (usually muscovite), and potassium feldspar (orthoclase or microcline). Sodium and potassium feldspars usually occur as distinct mineral grains, sizes varying from millimeter to meter scale. However, they are frequently intergrown, having exsolved from a single phase at high temperatures. If the intergrowth is visually observable in a hand specimen, the composite material is known as macroperthite; if visible only in a microscope, microperthite; and if submicroscopic in scale, cryptoperthite. In metamorphic rocks albite is found in granitic gneisses, and it may be the principal component of arkose, a feldspar-dominant, sedimentary rock. Cleavelandite, a platy variety, is sometimes found in lithium-rich pegmatites.