Orthoclase

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Optical properties - Orthoclase
Crystal System: Monoclinic
Color in PPL: Colorless, commonly with a dusty or cloudy appearance
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 (KAlSi3O8) is an important igneous rock forming tectosilicate mineral. It is also known as alkali feldspar and is common in granite and related rocks.

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 or red; rarely green. Twin crystals are not uncommon. Orthoclase is a common constituent of most granites and other felsic igneous rocks and is often found in huge crystals and masses in pegmatite veins. Orthoclase is used in the manufacture of porcelain and as a constituent of scouring powder. 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. A glassy kind of orthoclase, called sanidine, is typical of felsic volcanic rocks and is found in the trachytes of the Drachenfels, Germany.

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