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The mineral anorthoclase ((Na,K)AlSi3O8) is a crystalline solid solution in the
alkali feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldspa ...
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.Deere, Howie and Zussman, ''Rock Forming Minerals,'' Vol. 4 ''Framework Silicates,'' Wiley, pp. 2 - 5, Fig. 1


Structure and stability

Anorthoclase is an intermediate member of the high albitesanidine 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 400 to 600 °C only very limited solution exists (less than about 5% on both the low albite and
microcline Microcline (KAlSi3O8) is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium. It is common in granite and pegmatites. Microcline forms durin ...
ends).Klein, Cornelis and Cornelius S. Hurlbut, Jr. ''Handbook of Mineralogy,'' Wiley, pp. 446-449 (Fig. 11-95 Anorthoclase and high albite exhibit triclinic symmetry, whereas sanidine and the low temperature orthoclase have
monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...
symmetry. If the high temperature intermediate composition alkali feldspars are allowed to cool slowly, exsolution occurs and a
perthite Perthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar (near K-feldspar, KAlSi3O8, in composition) includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar (near albite, ...
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
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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,
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. The name is from the Greek αν, ορθός and κλάσις (not cleaving at right-angles), for its oblique cleavage.


See also

* Erebus crystal


References

{{Reflist Feldspar Triclinic minerals Minerals in space group 2