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Orthoclase

A variety of feldspar, essentially potassium aluminum silicate, KAlSi₃O₈, characterized by a monoclinic crystalline structure and found in igneous or granitic rock. Also called potash feldspar.

Common alkali feldspar mineral, potassium aluminosilicate (KAlSi₃O₈), that usually occurs as variously coloured grains in granite. Orthoclase is used in the manufacture of glass and ceramics; occasionally, transparent crystals are cut as gems. It is primarily important as a rock-forming mineral, however, and is abundant in igneous rocks, pegmatites, and gneisses(banded or foliated metamorphic rocks, usually of the same composition as granites). The feldspar minerals consist of sodium, potassium, and calcium aluminosilicates, and any feldspar may be chemically classed by the percentage of each of these three pure compounds, called end-members. Orthoclase is the potassium-bearing end-member of the system. Microcline, a mineral of the feldspar group, chiefly KAlSiO, used in making glass, porcelain, and enamel, is a lower temperature structural form of the same chemical composition as orthoclase.

Material Notes:
Two cleavage directions. Weathering end product(s): Kaolinite illite, the principal constituent of kaolin. An important rock forming mineral occuring in basic igneous (granite, syenite, granodiorite) and metamorphic. All feldspars are good indicators of the rock freshness.

Many igneous rocks contain orthoclase, which is more stable than sanidine(a glassy variety of orthoclase feldspar, known as moonstone when translucent) at lower temperatures. The compositional range of orthoclase extends only partway to albite(a widely distributed white feldspar, NaAlSiO, that is one of the common rock-forming plagioclase group), and orthoclase may occur with albite in rocks. In some igneous rocks and in most metamorphic rocks, microcline is the common potassium feldspar. Microcline can accommodate only a little sodium and, like orthoclase, may occur with albite.

In igneous rocks orthoclase is common and may sometimes, when white, be confused with Na-plagioclase.
Orthoclase forms an essential constituent of many acidic igneous rocks (granite, syenite, porphyry, trachyte(a light-colored igneous rock consisting essentially of alkali feldspar), phonolite(a light-colored volcanic rock composed largely of feldspars), &c.) and of crystalline schists and gneisses. In porphyries and in some granites (e.g. those of Shap in Westmorland, Cornwall, &c.) it occurs as embedded crystals with well-defined outlines, but usually it presents no crystalline form.

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