orthoclase

n.
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.

[Greek ortho-, ortho- + Greek klasis, a breaking (from klān, to break).]

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Potassium feldspar (Or = KAlSi₃O₈) that usually contains up to 30 mole % albite (Ab = NaAlSi₃O₈) in solid solution. Its hardness is 6; specific gravity, 2.57–2.5, depending on Ab content; mean refractive index, 1.52; color, white to dull pink or orange-brown. Some orthoclases may be intergrown with relatively pure albite which exsolved during cooling from a high temperature in pegmatites, granites, or granodiorites. This usually is ordered low albite, but in rare cases it may show some degree of Al,Si disorder, requiring it to be classified as analbite or high albite. If exsolution is detectable by eye, the Or-Ab composite mineral is called perthite; if microscopic examination is required to distinguish the phases, it is called microperthite; and if exsolution is detectable only by x-ray diffraction or electron optical methods, it is called cryptoperthite. Orthoclase is optically monoclinic. Its structure averaged over hundreds of nanometers may be monoclinic, but its true symmetry is triclinic. See also Albite; Crystal structure; Perthite.

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KAlSi
Monoclinic -- prismatic

Environment

A mineral of igneous, plutonic, and metamorphic rocks, and occasionally of high-temperature veins.

Crystal description

Isolated, 1-in. (3-4 cm) sharp white singles sometimes weather from porphyry dikes (Good Springs, Nevada, etc.) where they can be collected on the outcrop of a crumbling granulating rock. The best crystals are found in porphyries. Often they are elongated on the a -axis, parallel to the base, so the upright (front and back) prism pairs are rather short. Others may be flattened on the side, having large b -faces paralleling the a - c axes. With crystals of this habit, penetrant intergrowths of 2 individuals, Carlsbad twins, are common. Twins are named according to localities where they were first conspicuous: Carlsbad (Czechoslovakia) has the pictured pair side by side and halfway through, with a tent-top of one emerging from the base of the other; Baveno (Italy) twins are more square, as the twin plane is a dome that cuts diagonally across the pair; Manebach (Germany) twins show a re-entrant angle at the twin plane junction, which is the c -face. In sanidine the base is usually the largest face, while in adularia, as a rule, prisms dominate.

Physical properties

White, flesh, yellow, brown, colorless. Luster glassy (sanidine), translucent (adularia) to porcelaneous; hardness 6; specific gravity 2.6; fracture irregular; cleavage 2 good pinacoidal at 90°, plus occasional prismatic. Brittle; transparent to practically opaque.

Composition

Potassium aluminum silicate (16.9% K ₂ O 18.4% Al ₂ O ₃ , 64.7% SiO ₂ ). Sodium can replace up to 50% of the potassium in sanidine.

Tests

Fusible only with some difficulty, insoluble in acids. Sanidine chip may glow blue-white in gas flame but colors the flame only slightly. Fuses only on thin edges. Nonfluorescent even after roasting.

Distinguishing characteristics

A common mineral that resembles several other silicates, but it may usually be distinguishable from spodumene by its blocky cleavage (as opposed to splintery), from the plagioclases by its lack of twin striations on the good cleavage face, and by its 90° cleavages. The hardness and acid insolubility distinguish it from calcite, and its slight fusibility from amblygonite.

Occurrence

As a constituent of aplite (a granite composed exclusively of orthoclase and quartz), orthoclase is used in the ceramic and glass industry (see microcline, p. 279). Transparent varieties have slight gem use. Much microcline has been called orthoclase. Although orthoclase is primarily a rock-making mineral of igneous or plutonic rocks, mineral specimens and free crystals usually come in veins and in porphyritic rocks.

The glassy variety known as sanidine forms tabular crystals embedded in volcanic rocks. It sometimes reflects a bluish sheen in certain crystal directions. This phenomenon is known as adularescence, and gems cut from such feldspar are moonstones. Good, though not commercial. sanidine and moonstone have come from New Mexico. A transparent yellow variety from a single unusual Madagascar pegmatite has been cut into brilliant jewelry stones.

Adularia forms 4-8 in. (10-20 cm) colorless to white prismatic crystals (most abundant in Switzerland, where they occur in cavities in the metamorphosed rocks, often partly coated with green chlorite grunge). The name comes from one locality, Adular.

Loose crystals of common orthoclase are best formed in phenocrysts in porphyritic granitic rock, from which it sometimes weathers -- as at Robinson, Colorado, and Goodsprings, Nevada -- providing collectors with fresh, model-like crystals. Some granites seem to have been attacked by late solutions, and many granite outcrops just separate into quartz-feldspar gravel. (Recemented as sediments, these become arkose.)

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