Anorthoclase

Feldspar Group. Anorthoclase is strictly a K-rich variety of albite. It was long used (and still is by petrologists) as an intermediate member of the high albite – sanidine (alkali feldspar) solid solution series, in which the albite member exists in larger proportion. It was defined based partly on optics, for compositions between 10 and 36 percent of KAlSi3O8 and between 64 and 90 percent of NaAlSi3O8. It occurs in high-temperature sodic volcanic and hypabyssal rocks. Members of this series (high albite, anorthoclase and sanidine) are stable at temperatures of 400 °C and above. Below 400°C only very limited solution exists (less than about 5% on both the low albite and microcline ends). Anorthoclase and high albite exhibit triclinic symmetry, whereas sanidine and the medium temperature orthoclase have monoclinic symmetry. If the high temperature, intermediate composition alkali feldspars are allowed to cool slowly, exsolution occurs and a perthite structure results. Compare Unnamed (New Ordered Member of the Alkali Feldspar Series) - an ordered K-Na feldspar.