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A large group of aluminum silicate minerals that compose at least 60% of the earth's crust. Feldspars are divided into three primary groups:

1) potassium aluminum silicates - orthoclase, microcline

2) sodium aluminum silicates- albite, anorthoclase

3) calcium aluminum silicates - anorthite

Plagioclase feldspars contain sodium and calcium aluminum silicates while feldspathoids contain potassium and sodium aluminum silicates. A few of the feldspars are found as gemstones such as moonstone, sunstone, and Amazon stone, but most are found in mineral structures (granite, diorite). They are used the production of clays (kaolin), ceramics, glass, concrete, and abrasives. Feldspars act as a flux in ceramics, glazes, and glass by lowering the fusing point.

Orthoclase (potassium feldspar)

Synonyms and Related Terms

moonstone; sunstone; Amazone stone; granite; diorite; kaolin; feldspathic rock; feldespato (Esp.); feldspath (Fr.); feldspato (Port.); Feldspat (Deut.); veldspaat (Ned.)

Other Properties

Low birefringence.

Mohs Hardness 6.0 - 6.5
Melting Point 1250-1450
Density 2.55-2.75

Hazards and Safety

No significant hazards from ingestion or skin contact. Inhalation may cause silicosis if stone contains free silica.

Microcline (potssium feldspar )


Properties of Common Gemstones

Additional Images


  • Ralph Mayer, Ralph Mayer, A Dictionary of Art Terms and Techniques, Harper and Row Publishers, New York, 1969 (also 1945 printing)
  • Walter C. McCrone, John Gustave Delly, Walter C. McCrone, John Gustave Delly, The Particle Atlas, W. McCrone Associates, Chicago, IV, 1972
  • Michael McCann, Michael McCann, Artist Beware, Watson-Guptill Publications, New York City, 1979
  • Jack Odgen, Jack Odgen, Jewellery of the Ancient World, Rizzoli International Publications Inc., New York City, 1982
  • Robert Fournier, Robert Fournier, Illustrated Dictionary of Practical Pottery, Chilton Book Company, Radnor, PA, 1992
  • CRC Handbook of Chemistry and Physics, Robert Weast (ed.), CRC Press, Boca Raton, Florida, v. 61, 1980 Comment: density=2.55-2.75

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