ANORTHOCLASE, the name given by Rosenbusch in 1885 to include triclinic potash-soda felspars having cleavage angles varying but slightly from 9o° (87°-89° 3o'). The form develop ment is variable, like that of orthoclase (q.v.), but the best known crystals have a characteristic habit in which only the forms (Irt), (I io) and (aoi) are developed, giving rise to lozenge or rhomb-shaped individuals. Anorthoclase is probably to be regarded as a solid solution of the molecules and with small amounts of in which the sodium compound predominates. Most crystals fall within the composi tion limits and (Or=orthoclase Ab=albite).
Like the plagioclases, anorthoclase is twinned on the Carlsbad, albite and pericline laws, but in the case of the multiple twins the lamellae are characteristically very narrow. Optically anortho clase is distinguished by its moderate optic axial angle (2V=43° and is separated from all plagioclases but albite by its low refractive indices, and from albite by its optically negative character. The mineral is confined to alkaline igneous rocks; in the larvikites and foyaitic rocks of Norway and Madagascar, and as a prominent constituent of the rhomb-porphyry lavas of Nor way, of the alkaline lavas of the island of Pantelleria (south of Sicily), in the Kenyte lavas of Mt. Kenya (British East Africa) and Mt. Erebus (Antarctica). (C. E. T.)