The mineral anorthoclase ((Na,K)AlSi3O8) is a crystalline solid solution in the alkali feldspar series, in which the sodium-aluminium silicate member exists in larger proportion.

[5] Anorthoclase is an intermediate member of the high albite – sanidine alkali feldspar solid solution series.

[6] If the high temperature intermediate composition alkali feldspars are allowed to cool slowly, exsolution occurs and a perthite structure results.

[6] Anorthoclase occurs in high temperature sodium rich volcanic and hypabyssal (shallow intrusive) rocks.

The mineral is typically found as a constituent of the fine grained matrix or as small phenocrysts which may occur as loose crystals in a weathered rock.