Orthoclase

orthoclase

Orthoclase from Serra de Peneda, Portugal

(credit: Emil Javorsky/EB Inc.)

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. 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 is a lower temperature structural form of the same chemical composition as orthoclase.

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orthoclase [′ȯr·thə‚klās]

(mineralogy)

KAlSi₃O₈A colorless, white, cream-yellow, flesh-reddish, or gray potassium feldspar that usually contains some sodium feldspar, either as albite or analbite or in some intermediate state; it is or appears to be monoclinic. Also known as common feldspar; orthose; pegmatolite.

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Orthoclase 

a rock-forming mineral of the feldspar group with the chemical composition K[AlSi₃O₈]. Orthoclase contains admixtures of Na (up to 8 percent Na₂O); less frequently, it contains Ba and small amounts of Fe, Ca, Rb, and Cs. It crystallizes in the monoclinic system to form prismatic crystals. Twin crystals are characteristic. Orthoclase, unlike microcline, exhibits perfect cleavage at a 90° angle (hence the name “orthoclase”). It is light pink, brownish yellow, or red. The mineral has a vitreous luster, a hardness on Mohs’ scale of 6–6.5, and a density of 2,550–2,580 kg/m³.

Orthoclase is one of the principal rock-forming minerals in magmatic rocks. Clusters of large orthoclase crystals occur in pegmatite veins. The mineral is often formed during regional and contact metamorphism. Orthoclase is used in the manufacture of glassware and ceramics.