ANORTHITE, the calcium end member of the plagioclase (q.v.) group of felspars (from the Gr. ay, privative and opOos, upright, in allusion to its oblique crystals). Anorthite crystal lizes in the triclinic system, and has the chemical composition CaAlSi.,08• Perfectly pure anorthite is unknown in nature, but is readily synthesized. The name is usually reserved for those solid solutions ranging in composition from pure anorthite to a mixture containing io% of the albite molecule. Such anorthite occurs in some gabbros and basalts, and also in contact metamorphosed limestones.