13 Photos
Anorthoclase
Chemical Formula	(Na,K)AlSi3O8
Species	Silicates
Crystal System	Triclinic
Mohs Scale	6
Specific Gravity	2.57-2.60
Color	Colorless, also white, pale creamy yellow, red, green
Streak	White
Luster	Vitreous to pearly on cleavage planes
Refractive Index	n = 1.519 - 1.529 n = 1.524 - 1.534 n = 1.527 - 1.536
Diaphaneity	Transparent
Cleavage	Perfect
Fracture	Uneven
Crystal Habit：Short prismatic crystals; tabular, rhombic, flattened along

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. It typically consists of between 10 and 36 percent of KAlSi₃O₈ and between 64 and 90 percent of NaAlSi₃O₈.

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 600 °C and above. Below 400 to 600 °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 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.

Occurrence

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.

It was first described in 1885 for an occurrence on Pantelleria Island, Trapani Province, Sicily. The name is from the Greek αν, ορθ?? and κλ?σι?, for its oblique cleavage.