orthoclase

/awr"theuh klays', -klayz'/, n.

a common white or pink mineral of the feldspar group, KAlSi₃O₈, having two good cleavages at right angles, and found in silica-rich igneous rocks: used in the manufacture of porcelain.

[1840-50; ORTHO- + -clase < Gk klásis cleavage, breaking]

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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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▪ mineral

 common alkali feldspar mineral, a potassium aluminosilicate (KAlSi₃O₈); it usually occurs as variously coloured, frequently twinned crystals in granite. Orthoclase is used in the manufacture of glass and ceramics; occasionally, transparent crystals are cut as gems. Orthoclase is primarily important as a rock-forming mineral, however, and is abundant in alkali and acidic igneous rocks, in pegmatites, and in gneisses. For detailed physical properties, see feldspar (table).

      The feldspar minerals are mixtures of sodium, potassium, and calcium aluminosilicates, and any feldspar may be classed by its percentage of each of these three pure compounds, called end-members. Orthoclase is the potassium-bearing end-member of the system; its symbol is Or.

      Orthoclase and the sodium-bearing end-member of the system, albite (sodium aluminosilicate; NaAlSi₃O₈), form the alkali feldspars, a series in which sodium-bearing and potassium-bearing species intermingle; thus, there is a continuous chemical variation between the two end-members. Because the intermediate members, called orthoclase–microperthites, cannot blend homogeneously, they take the form of intergrowths of microscopic but distinct crystals of the sodium and potassium end-members.

      Orthoclase has an intermediate degree of ordering of the aluminum and silicon atoms in its aluminosilicate crystal framework, falling between the fully ordered arrangement of microcline and the random arrangement of high-sanidine.

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