What is "anorthoclase"

Anorthoclase \A*nor"tho*clase\, n. [Gr. ? priv. + orthoclase.] (Min.) A feldspar closely related to orthoclase, but triclinic. It is chiefly a silicate of sodium, potassium, and aluminium. Sp. gr., 2.57 -- 2.60. [Webster 1913 Suppl.] ||

n. (context mineral English) Any of a group of feldspars being mixed sodium and potassium aluminosilicates.

The mineral anorthoclase ((Na,K)AlSiO) is a crystalline solid solution in the alkali feldspar series, in which the sodium-aluminium silicate member exists in larger proportion. It typically consists of between 10 and 36 percent of KAlSiO and between 64 and 90 percent of NaAlSiO.