orthoclase

{{Infobox mineral | name = Orthoclase | category = Tectosilicate minerals | group = Feldspar group | series = Alkali feldspar series | boxwidth = | boxbgcolor =#c6a289 | boxtextcolor = #fff | image = OrthoclaseBresil.jpg | imagesize = 260 | caption = | formula = KAlSi3O8 | IMAsymbol = Or | IMAstatus = Grandfathered (1801) | molweight = | strunz = 9.FA.30 | dana = 76.1.1.1 | system = Monoclinic | class = Prismatic (2/m) | symmetry = C2/m (no. 12) | color = Colorless, greenish, greyish yellow, white, pink | 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 | tenacity = | mohs = 6 (defining mineral) | luster = Vitreous, pearly on cleavage surfaces | polish = | refractive = nα = 1.518–1.520 nβ = 1.522–1.524 nγ = 1.522–1.525 | opticalprop = Biaxial (−), 2V = 65–75 | birefringence = 0.0050–0.0060 | dispersion = Relatively strong | pleochroism = | length fast/slow = No slow or fast length | extinction = Parallel to cleavage | fluorescence= | absorption = | streak = White | gravity = 2.55–2.63 | density = | melt = | fusibility = | diagnostic = Distinguishable from microcline by a lack in gridiron twinning. Distinguishable from sanidine by a larger 2Vx. | solubility = | diaphaneity = Transparent to translucent | other = Low negative relief;alters to sericite or clay (commonly) | references = }}

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 alkali feldspar, also known as potassium feldspar or K-spar. The gem known as moonstone (see below) is largely composed of orthoclase.