The mineral anorthoclase ((Na,K)AlSi
3O
8) 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 KAlSi
3O
8 and between 64 and 90 percent of NaAlSi
3O
8.
[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
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 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 matrix
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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, ]Sicily
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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