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CategorySilicate meeneral
(repeatin unit)
Creestal seestemMonoclinic (2/m)
space group C2/m
ColourColourless, Greenish, Greyish yellae, White, Pink
Creestal habitCan be anhedral or euhedral. Grains are commonly elongate wi a tabular appearance.
TwinninTypically displays carlsbad twinnin. Baveno an manebach twins hae an aa been reportit in orthoclase.
CleavageHaes 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 haurdness6 (definin mineral)
SkinkleVitreous, pearly on cleavage surfaces
DiaphaneityTransparent tae translucent
Speceefic gravity2.55–2.63
Optical propertiesBiaxial (-), 2V = 65–75
Refractive indexnα = 1.518–1.520
nβ = 1.522–1.524
nγ = 1.522–1.525
Dispersionrelatively strang
Extinctionparallel tae cleavage
Diagnostic featursDistinguishable frae microcline bi a lack in gridiron twinnin. Distinguishable frae sanidine bi a lairger 2Vx.
Ither chairacteristicsLaw negative relief

Orthoclase (endmember formula KAlSi3O8) is an important tectosilicate mineral which furms igneous rock. The name is frae the Greek for "straight fracture," acause its twa cleavage planes are at richt angles tae ilk 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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