anorthoclase

The mineral anorthoclase ((Na,K)AlSi3O8) is a crystalline solid solution in the alkali feldspar series, in which the proportion of the sodium-aluminium silicate member exceeds the potassium-aluminium silicate member. It typically consists of between 64 and 90 percent of NaAlSi3O8 and between 10 and 36 percent of KAlSi3O8. thumb|left|upright|An anorthoclase crystal from Mount Erebus, Antarctica

==Structure and stability== Anorthoclase is an intermediate member of the high albite – sanidine alkali feldspar solid solution series. Intermediate members of this series, high albite, anorthoclase and high sodium sanidine are stable at temperatures of and above. Below 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 low 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.