PLAGIOCLASE

any member of the series of abundant feldspar minerals usually occurring as light-coloured, glassy, transparent to translucent, brittle crystals. Plagioclase is a mixture of albite (Ab), or sodium aluminosilicate (NaAlSi3O8), and anorthite (An), or calcium aluminosilicate (CaAl2Si2O8); the two intermingle and form a continuous chemical range (called a solid-solution series) between albite and anorthite (the end-members of the series). Intermediate members are homogeneous mixtures of the pure end-members. Divided according to their composition, the members of the plagioclase series are named in the Table. Most of the crystals of the members of this series exhibit microscopic, repeated lamellar albite twinning; this produces the fine parallel striations observed on some surfaces. The twinning sometimes causes an iridescence, usually blue or green, that may arise either from reflection or diffraction at the edges of the lamellae, or from diffusion by adjoining areas with different optical properties. Plagioclase is used in the manufacture of glass and ceramics; the iridescent varieties peristerite and labradorite are valued as gemstones or ornamental material. The primary importance of plagioclase, however, derives from its role in rock formation. Oligoclase, the most common plagioclase, occurs in granite, diorite, and other acidic igneous rocks and in some metamorphic rocks; notable occurrences are at Aust-Agder, Norway, and Fine, N.Y., United States. Andesine, less common, occurs in many granular and volcanic rocks with medium silica content, as in Marmato, Colom., and Bodenmais, Bavaria, Ger. The rarest plagioclase is bytownite, which occurs in basic igneous rocks and in stony meteorites. For detailed physical properties of the plagioclase series, see feldspar (table).