orthoclase

View more Science videos

(click to enlarge)
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.

For more information on orthoclase, visit Britannica.com.

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.

Environment

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

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.

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.

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.

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.

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.

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.)

Orthoclase
General
Category	Silicate mineral
Chemical formula	KAlSi3O8
Identification
Color	Colorless, Greenish, Grayish yellow, White, Pink
Crystal habit	Can be anhedral or euhedral. Grains are commonly elongate with a tabular appearance.
Crystal system	Monoclinic (2/m) space group C2/m
Twinning	Typically displays carlsbad twinning. Baveno and manebach twins have also been reported in orthoclase.
Cleavage	Has perfect cleavage on {001} and good cleavage on {010}. Cleavages intersect at 90°. It can be difficult to see cleavage in thin section due to orthoclase's low relief.
Mohs scale hardness	6 (defining mineral)
Luster	Vitreous, pearly on cleavage surfaces
Streak	white
Diaphaneity	Transparent to translucent
Specific gravity	2.55–2.63
Optical properties	Biaxial (-), 2V = 65–75
Refractive index	nα = 1.518–1.520 nβ = 1.522–1.524 nγ = 1.522–1.525
Birefringence	0.0050–0.0060
Dispersion	relatively strong
Extinction	parallel to cleavage
Diagnostic features	Distinguishable from microcline by a lack in gridiron twinning. Distinguishable from sanidine by a larger 2Vx.
Other characteristics	Low negative relief
References	[1][2][3]

Orthoclase (endmember formula KAlSi₃O₈) is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar. The gem known as moonstone (see below) is largely composed of orthoclase.

Contents 1 Formation and subtypes 2 Uses 3 See also 4 References

Formation and subtypes

Orthoclase is a common constituent of most granites and other felsic igneous rocks and often forms huge crystals and masses in pegmatite.

Typically, the pure potassium endmember of orthoclase forms a solid solution with albite, the sodium endmember (NaAlSi₃O₈), of plagioclase. While slowly cooling within the earth, sodium-rich albite lamellae form by exsolution, enriching the remaining orthoclase with potassium. The resulting intergrowth of the two feldspars is called perthite.

Adularia with pyrite incrustations.

The higher-temperature polymorph of orthoclase is sanidine. Sanidine is common in rapidly cooled volcanic rocks such as obsidian and felsic pyroclastic rocks, and is notably found in trachytes of the Drachenfels, Germany. The lower-temperature polymorph of orthoclase is microcline. Adularia is found in low temperature hydrothermal deposits, in the Adula Alps of Switzerland. The largest documented single crystal of orthoclase was found in Ural mountains, Russia. It measured ~10×10×0.4 m³ and weighed ~100 tons.^[4]

Uses

Together with the other potassium feldspars orthoclase is a common raw material for the manufacture of some glasses, some ceramics, such as porcelain, and as a constituent of scouring powder.

Some intergrowths of orthoclase and albite have an attractive pale luster and are called moonstone when used in jewelery. Most moonstones are translucent and white, although grey and peach-colored varieties also occur. In gemology, their luster is called adularescence and is typically described as creamy or silvery white with a "billowy" quality. It is the state gem of Florida.

The gemstone commonly called rainbow moonstone is more properly a colorless form of labradorite and can be distinguished from "true" moonstone by its greater transparency and play of color, although their value and durability do not greatly differ.

See also

• List of minerals

Wikimedia Commons has media related to: Orthoclase

References

1. ^ Mindat
2. ^ Handbook of Mineralogy
3. ^ Webmineral
4. ^ P. C. Rickwood (1981). "The largest crystals". American Mineralogist 66: 885–907. http://www.minsocam.org/ammin/AM66/AM66_885.pdf. 

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)