Orthoclase

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Orthoclase
OrthoclaseBresil.jpg
General
Category Silicate mineral
Formula
(repeating unit)
KAlSi3O8
Identification
Color Colourless, Greenish, Greyish yellae, White, Pink
Crystal habit Can be anhedral or euhedral. Grains are commonly elongate wi a tabular appearance.
Crystal seestem Monoclinic (2/m)
space group C2/m
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 hairdness 6 (definin mineral)
Luster Vitreous, pearly on cleavage surfaces
Streak white
Diaphaneity Transparent tae 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 strang
Extinction parallel tae cleavage
Diagnostic features Distinguishable frae microcline bi a lack in gridiron twinnin. Distinguishable frae sanidine bi a lairger 2Vx.
Ither characteristics Law negative relief
References [1][2][3]

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 to each other. Alternate names are alkali feldspar, potassium feldspar an K-feldspar. The gem kent as muinstane is lairgely componed o orthoclase.

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