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Orthoclase (potassium feldspar)


A common type of Feldspar composed of potassium aluminum silicate. Orthoclase is found in many igneous rocks worldwide. It is generally a white or pale pink in color and has two good cleavage planes at right angles. Pure orthoclase is called adularia. Orthoclase with an intergrowth of albite has a pearly opalescence and is called Moonstone. It is used as a gemstone. Sanidine is a glassy type of orthoclase. Orthoclase has been used since ancient times for beads. It is also used in the manufacture of glass, porcelain and enamels.


Synonyms and Related Terms

potash feldspar; adularia; moonstone; sanidine; microcline; amazonstone; amazonite; ortoclasa (Esp.); orthoclase(Fr.); ortose (Port.); Orthoklas (Deut.); orthoclaas (Ned.)





Physical and Chemical Properties

Monoclinic system; twinned crystals common. Perfect cleavage in two directions (right angles). Fracture = uneven. Luster = vitreous. Streak = white.

Composition KAlSi3O8
Mohs Hardness 6.0 - 6.5
Density 2.56-2.58
Refractive Index 1.52


Properties of Common Gemstones

Resources and Citations

  • Jack Odgen, Jewellery of the Ancient World, Rizzoli International Publications Inc., New York City, 1982
  • C.W.Chesterman, K.E.Lowe, Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf, New York, 1979
  • G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p. 316
  • Van Nostrand's Scientific Encyclopedia, Douglas M. Considine (ed.), Van Nostrand Reinhold, New York, 1976
  • Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
  • The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998