Orthoclase

Orthoclase
General
CategorySilicate mineral
Formula
(repeating unit)
KAlSi3O8
IMA symbolOr[1]
Strunz classification9.FA.30
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H–M symbol)
Space groupC2/m
Identification
ColorColorless, greenish, greyish yellow, white, pink
Crystal habitCan be anhedral or euhedral. Grains are commonly elongate with a tabular appearance.
TwinningTypically displays Carlsbad twinning. Baveno and manebach twins have also been reported in orthoclase.
CleavageHas 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.
FractureUneven
Mohs scale hardness6 (defining mineral)
LusterVitreous, pearly on cleavage surfaces
StreakWhite
DiaphaneityTransparent to translucent
Specific 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
Birefringence0.0050–0.0060
DispersionRelatively strong
ExtinctionParallel to cleavage
Length fast/slowNo slow or fast length
Diagnostic featuresDistinguishable from microcline by a lack in gridiron twinning. Distinguishable from sanidine by a larger 2Vx.
Other characteristicsLow 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.

  1. ^ Warr, L. N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ "Orthoclase: Orthoclase mineral information and data". www.mindat.org. Retrieved 17 April 2018.
  3. ^ "Handbook of Mineralogy" (PDF).
  4. ^ Barthelmy, Dave. "Orthoclase Mineral Data". www.webmineral.com. Retrieved 17 April 2018.