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Optical properties
Crystal System: Monoclinic
Color in PPL: Colorless, commonly with a dusty or cloudy appearance; white to pink in hand-sample
Pleochroism: N/A
Habit/Shape: Can be anhedral or euhedral. Grains are commonly elongate with a tabular appearance.
Relief: Low negative relief
Cleavage/Fracture Habit: 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.
Twinning: Typically displays carlsbad twinning. Baveno and manebach twins have also been reported in orthoclase.
Birefringence: Up to first order white (roughly 0.007)
Extinction Habit/Angle: Parallel extinction to cleavage
Length Slow/Fast: N/A
Optic Sign: Biaxial negative
2Vx: 40-70°
Alteration: Commonly alters to sericite or clay.
Distinguishing Characteristics: Distinguishable from microcline by a lack in gridiron twinning. Distinguishable from sanidine by a larger 2Vx.

Orthoclase (endmember formula KAlSi3O8) is an important tectosilicate mineral, which forms igneous rock. It is also known as alkali feldspar and is common in granite and related rocks.

Additional recommended knowledge

Orthoclase is named based on the Greek for "straight fracture," because its two cleavages are at right angles to each other. Orthoclase crystallizes in the monoclinic crystal system. It has a hardness of 6, a specific gravity of 2.56-2.58, and a vitreous to pearly luster. It can be colored white, gray, yellow, pink, or red; rarely green. Twinned crystals are quite common. Orthoclase is a common constituent of most granites and other felsic igneous rocks and is often found in huge crystals and masses in pegmatite masses.

Typical orthoclase is a solid solution between the pure potassium endmember and the sodium endmember, albite (NaAlSi3O8). During slow cooling within the earth, sodium-rich albite lamellae form by exsolution and the remaining orthoclase becomes more potassium-rich. The resulting intergrowth of the two feldspars is called perthite.

The higher-temperature polymorph of orthoclase is sanidine. Sanidine is common in rapidly cooled volcanic rocks, such as obsidian and felsic pyroclastic rocks. A notable locality with sanidine is in the trachytes of the Drachenfels, Germany. The lower-temperature polymorph of orthoclase is microcline.

Adularia (from Adular) is found in low temperature hydrothermal deposits. When pearly and opalescent, orthoclase is called moonstone and is used in jewelry. These opalescent varieties are known to be an intergrowth of orthoclase and albite.

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.

It is the state gem of Florida.  

See also


  • Alkali feldspars U. Texas
  • Mindat
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Orthoclase". A list of authors is available in Wikipedia.
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