microcline

n.
A mineral of the feldspar group, chiefly KAlSi₃O₈, used in making glass, porcelain, and enamel.

[Greek mīkro-, micro- (from the fact that its cleavage angle is not exactly equal to 90°) + Greek klīnein, to lean; see cline.]

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Triclinic potassium feldspar, KAlSi₃O₈, that usually contains a few percent sodium feldspar (Ab = NaAlSi₃O₈) in solid solution. Its hardness is 6; specific gravity, 2.56; mean refractive index, 1.52; color, white (green varieties are called amazon stone or amazonite). Microcline is found in some relatively high-grade regional metamorphic rocks, but is much more common in pegmatities, granites, and related plutonic igneous rocks. In the last, it often occurs as a microcline perthite, containing exsolved low albite intergrowths. See also Feldspar; Perthite.

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KAlSi
Triclinic -- pinacoidal

Environment

Usually found in granite pegmatites, where it takes the place of orthoclase, and in many granitic plutonic rocks as well.

Crystal description

Crystals common, essentially the same as orthoclase and larger; any inclination of the b -axis toward triclinicism is too slight to really register. Often large, frequently twinned like orthoclase. The inclination of the third axis is only a few minutes, so the forms are best likened to monoclinic crystals.

Physical properties

White, flesh, red-brown, green. Luster glassy to porcelaneous; hardness 6; specific gravity 2.5-2.6; fracture irregular; cleavage like orthoclase, 2 good pinacoidal and almost at right angles, and poor prismatic. Brittle; translucent to pretty opaque.

Composition

Potassium aluminum silicate, the same as orthoclase (preceding).

Tests

The hardness and the cleavage are usually sufficient for identification, when considered in relation to the associated minerals.

Distinguishing characteristics

Microcline is the only bright green feldspar. When of another hue it is distinguished from orthoclase of granite in thin sections by a sort of Scottish plaid pattern under crossed Nicol prisms (which produce plane-polarized light). In granitic pegmatite occurrences, one assumes the feldspar is microcline (pegmatite orthoclase is almost unknown). Distinguished from determinedly triclinic plagioclases by a lack of twin striations on the prominent cleavage faces.

Occurrence

Microcline is the characteristic feldspar of granite pegmatites (and often of ordinary granite). It may grow into tremendous crystals, several feet (a meter) on an edge. Good crystals may be obtained from open cavities or be broken free from massive quartz, which is easily chipped away. A green variety is sometimes used in jewelry under the name amazonstone. Microcline is quarried for ceramics, ceramic glazes, and for a scouring powder.

The best green crystals have come from scattered pegmatitic pockets in schist near Crystal Peak, near Colorado Springs, Colorado, associated with smoky quartz. Good green crystal chunks suitable for cutting were found at Amelia, Virginia, and in Brazil, India, Russia, and Madagascar. Ordinary microcline is the feldspar of almost every pegmatite.

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