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Category Silicate meeneral
(repeatin unit)
Creestal seestem Monoclinic (2/m)
space group C2/m
Colour Colourless, Greenish, Greyish yellae, White, Pink
Creestal habit Can be anhedral or euhedral. Grains are commonly elongate wi a tabular appearance.
Twinnin Typically displays carlsbad twinnin. Baveno an manebach twins hae an aa been reportit in orthoclase.
Cleavage Haes perfect cleavage on {001} an guid cleavage on {010}. Cleavages intersect at 90°. It can be difficult tae see cleavage in thin section due tae orthoclase's law relief.
Mohs scale haurdness 6 (definin mineral)
Skinkle Vitreous, pearly on cleavage surfaces
Streak white
Diaphaneity Transparent tae translucent
Speceefic 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 strang
Extinction parallel tae cleavage
Diagnostic featurs Distinguishable frae microcline bi a lack in gridiron twinnin. Distinguishable frae sanidine bi a lairger 2Vx.
Ither chairacteristics Law negative relief
References [1][2][3]

Orthoclase (endmember formula KAlSi3O8) is an important tectosilicate meeneral which furms igneous rock. The name is frae the Greek for "straight fracture," acause its twa cleavage planes are at richt angles tae each ither. Alternate names are alkali feldspar, potassium feldspar an K-feldspar. The gem kent as muinstane is lairgely componed o orthoclase.

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