Category Silicate mineral
Chemical formula KAlSi3O8
Color Colorless, Greenish, Grayish yellow, White, Pink
Crystal habit Can be anhedral or euhedral. Grains are commonly elongate with a tabular appearance.
Crystal system Monoclinic (2/m)
space group C2/m
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.
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
Diagnostic features Distinguishable from microcline by a lack in gridiron twinning. Distinguishable from sanidine by a larger 2Vx.
Other characteristics Low negative relief
References [1][2][3]

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


Formation and subtypes

Orthoclase is a common constituent of most granites and other felsic igneous rocks and often forms huge crystals and masses in pegmatite.

Typically, the pure potassium endmember of orthoclase forms a solid solution with albite, the sodium endmember (NaAlSi3O8), of plagioclase. While slowly cooling within the earth, sodium-rich albite lamellae form by exsolution, enriching the remaining orthoclase with potassium. The resulting intergrowth of the two feldspars is called perthite.

Adularia with pyrite incrustations.

The higher-temperature polymorph of orthoclase is sanidine. Sanidine is common in rapidly cooled volcanic rocks such as obsidian and felsic pyroclastic rocks, and is notably found in trachytes of the Drachenfels, Germany. The lower-temperature polymorph of orthoclase is microcline. Adularia is found in low temperature hydrothermal deposits, in the Adula Mountains of Switzerland. The largest documented single crystal of othoclase was found in Ural mountains, Russia. It measured ~10×10×0.4 m3 and weighed ~100 tons.[4]


Together with the other potassium feldspars orthoclase is a common raw material for the manufacture of some glasses, some ceramics, such as porcelain, and as a constituent of scouring powder.

Some intergrowths of orthoclase and albite have an attractive pale lustre and are called moonstone when used in jewellery. Most moonstones are translucent and white, although grey and peach-colored varieties also occur. In gemology, their lustre is called adularescence and is typically described as creamy or silvery white with a "billowy" quality. It is the state gem of Florida.

The gemstone commonly called rainbow moonstone is more properly a colourless form of labradorite and can be distinguished from "true" moonstone by its greater transparency and play of colour, although their value and durability do not greatly differ.

See also


  1. ^ Mindat
  2. ^ Handbook of Mineralogy
  3. ^ Webmineral
  4. ^ P. C. Rickwood (1981). "The largest crystals". American Mineralogist 66: 885–907. http://www.minsocam.org/ammin/AM66/AM66_885.pdf. 

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Look at other dictionaries:

  • Orthoclase — Or tho*clase, n. [Ortho + Gr. ? to break.] (Min.) Common or potash feldspar crystallizing in the monoclinic system and having two cleavages at right angles to each other. See {Feldspar}. [1913 Webster] …   The Collaborative International Dictionary of English

  • orthoclase — [ôr′thō klās΄, ôr′thōklāz΄; ôr′thəklās΄, ôr′thəklāz΄] n. [Ger orthoklas < Gr orthos (see ORTHO ) + klasis, fracture (< klan, to break: see CLASTIC), because of the 90° cleavage] a monoclinic feldspar, KAlSi3O8, that is dimorphic with… …   English World dictionary

  • orthoclase — /awr theuh klays , klayz /, n. a common white or pink mineral of the feldspar group, KAlSi3O8, having two good cleavages at right angles, and found in silica rich igneous rocks: used in the manufacture of porcelain. [1840 50; ORTHO + clase < Gk… …   Universalium

  • Orthoclase — Orthose Orthose Catégorie IX : silicates Orthose et quartz Généra …   Wikipédia en Français

  • orthoclase — noun Potassium aluminum silicate, KAlSiO, a common feldspar of igneous, plutonic, and metamorphic rocks. Orthoclase is the main feldspar of pegmatite occurrences, where it is most commonly flesh colored. Orthoclase is used in the ceramic and …   Wiktionary

  • orthoclase — ortoklazas statusas T sritis chemija apibrėžtis Mineralas. formulė K[AlSi₃O₈] atitikmenys: angl. orthoclase rus. ортоклаз …   Chemijos terminų aiškinamasis žodynas

  • orthoclase — noun Etymology: German Orthoklas, from orth + Greek klasis breaking, from klan to break more at clast Date: 1849 a monoclinic mineral of the feldspar group consisting of a silicate of potassium and aluminum …   New Collegiate Dictionary

  • orthoclase — [ ɔ:θəkleɪz] noun a potassium rich mineral of the feldspar group, typically white or pink. Origin C19: from ortho + Gk klasis breaking (because of the characteristic two cleavages at right angles) …   English new terms dictionary

  • orthoclase — n. Potash felspar, orthose, prismatic felspar, felspar …   New dictionary of synonyms

  • orthoclase — or·tho·clase …   English syllables

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