Orthoclase
General
Category	Silicate mineral
Formula (repeating unit)	KAlSi3O8
IMA symbol	Or[1]
Strunz classification	9.FA.30
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H–M symbol)
Space group	C2/m
Identification
Color	Colorless, greenish, greyish yellow, white, pink
Crystal habit	Can be anhedral or euhedral. Grains are commonly elongate with a tabular appearance.
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.
Fracture	Uneven
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
Length fast/slow	No slow or fast length
Diagnostic features	Distinguishable from microcline by a lack in gridiron twinning. Distinguishable from sanidine by a larger 2Vx.
Other characteristics	Low negative relief; alters to sericite or clay (commonly)
References	[2][3][4]

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

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.

The higher-temperature polymorph of KAlSi₃O₈ 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 KAlSi₃O₈ is microcline.

Adularia is a low temperature form of either microcline or orthoclase originally reported from the low temperature hydrothermal deposits in the Adula Alps of Switzerland.^[5] It was first described by Ermenegildo Pini in 1781.^[6] The optical effect of adularescence in moonstone is typically due to adularia.^[7]

The largest documented single crystal of orthoclase was found in the Ural Mountains in Russia. It measured around 10 m × 10 m × 0.4 m (33 ft × 33 ft × 1 ft) and weighed around 100 tonnes (110 short tons).^[8]

Together with the other potassium feldspars, orthoclase is a common raw material for the manufacture of some glasses and 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 jewelry. 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.

Orthoclase is one of the ten defining minerals of the Mohs scale of mineral hardness, on which it is listed as having a hardness of 6.

NASA's Curiosity rover discovery of high levels of orthoclase in Martian sandstones suggested that some Martian rocks may have experienced complex geological processing, such as repeated melting.^[9]

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