Orthoclase

Orthoclase
General
Category	Silicate mineral
Formula (repeating unit)	KAlSi3O8
IMA symbol	Or[1]
Strunz classification	9.FA.30
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H–M symbol)
Space group	C2/m
Identification
Color	Colorless, greenish, greyish yellow, white, pink
Crystal habit	Can be anhedral or euhedral. Grains are commonly elongate with a tabular appearance.
Twinning	Typically displays Carlsbad twinning. Baveno and manebach twins have also been reported in orthoclase.
Cleavage	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.
Fracture	Uneven
Mohs scale hardness	6 (defining mineral)
Luster	Vitreous, pearly on cleavage surfaces
Streak	White
Diaphaneity	Transparent to 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 strong
Extinction	Parallel to cleavage
Length fast/slow	No slow or fast length
Diagnostic features	Distinguishable from microcline by a lack in gridiron twinning. Distinguishable from sanidine by a larger 2Vx.
Other characteristics	Low negative relief; alters to sericite or clay (commonly)
References	[2][3][4]

Orthoclase, or orthoclase feldspar (endmember formula KAlSi₃O₈), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture", because its two cleavage planes are at right angles to each other. It is a type of potassium feldspar, also known as K-feldspar. The gem known as moonstone (see below) is largely composed of orthoclase.