Anorthite, CaAl2Si2O8, is classified as a plagioclase group feldspar tectosilicate, with 45 to 50% of the Si4+ in the tetrahedra framework substituted by Al3+. This large charge deficit is balanced mainly by the addition of Ca to the feldspar structure. Up to 10% of the Ca may be replaced by Na, in proportion to the total charge on the structure. Albite is the sodic end member of an isomorphous solid solution series, containing sodium and no calcium. Anorthite, the other end member, contains calcium and is one of the rarer members of the plagioclase series. The solid solution is not complete in that miscibility gaps exist. Anorthite exhibits a triclinic crystal system with prismatic structure.

Distribution

Anorthite is a widely distributed colorless, white, yellowish, green, or pink to reddish plagioclase feldspar. Anorthite is a rare feldspar and its only real use is as specimens for collectors. Found in igneous rocks and in contact metamorphic limestone, notable occurrences include Lake County, California; Franklin, New Jersey; Italy; and the earths moon.

Importance in Soils

In general, Feldspars may be a source of micronutrients as they weather. Not only rare, anorthite is the least stable of the feldspars with respect to weathering reactions.

Angel, R. J. 1988. High-pressure structure of anorthite. American Mineralogist. 73:1114-1119