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Title: Feldspar  
Author: World Heritage Encyclopedia
Language: English
Subject: Granite, Sandstone, Basalt, Geology of the Australian Capital Territory, Moonstone (gemstone)
Collection: Feldspar, Monoclinic Minerals, Tectosilicates, Triclinic Minerals
Publisher: World Heritage Encyclopedia


Feldspar crystal (18×21×8.5 cm) from Jequitinhonha valley, Minas Gerais, Southeastern Brazil.
Category tectosilicate
(repeating unit)
Color pink, white, gray, brown
Crystal system triclinic or monoclinic
Cleavage two or three
Fracture along cleavage planes
Mohs scale hardness 6.0-6.5
Luster Vitreous
Streak white
Diaphaneity opaque
Specific gravity 2.55 - 2.76
Density 2.56
Refractive index 1.518-1.526
Birefringence first order
Pleochroism none
Other characteristics exsolution lamellae common
References [1]
Compositional phase diagram of the different minerals that constitute the feldspar solid solution.

Feldspars (KAlSi3O8NaAlSi3O8CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up as much as 60% of the Earth's crust.[2]

Feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock.[3] Rock formed almost entirely of calcic plagioclase feldspar (see below) is known as anorthosite.[4] Feldspars are also found in many types of sedimentary rock.[5]


  • Etymology 1
  • Compositions 2
    • Alkali feldspars 2.1
    • Plagioclase feldspars 2.2
    • Barium feldspars 2.3
  • Production and uses 3
  • See also 4
  • References 5
  • Further reading 6


The name feldspar derives from the German Feldspat. The words Feld, "field", and Spath mean "a rock that does not contain ore." "Feldspathic" refers to materials that contain feldspar. The alternate spelling, felspar, has largely fallen out of use.[6]


This group of minerals consists of framework tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers:

Potassium-Feldspar (K-spar) endmember KAlSi3O8[2]

Albite endmember NaAlSi3O8[2]

Anorthite endmember CaAl2Si2O8[2]

Solid solutions between K-feldspar and albite are called alkali feldspar.[2] Solid solutions between albite and anorthite are called plagioclase,[2] or more properly plagioclase feldspar. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the earth. Albite is considered both a plagioclase and alkali feldspar. In addition to albite, barium feldspars are also considered both alkali and plagioclase feldspars. Barium feldspars form as the result of the replacement of potassium feldspar.

Alkali feldspars

The alkali feldspars are as follows:

Sanidine is stable at the highest temperatures, and microcline at the lowest.[7][8] Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye.[10] Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.

Plagioclase feldspars

The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses):

First X-ray view of Martian soil - feldspar, pyroxenes, olivine revealed (Curiosity rover at "Rocknest", October 17, 2012).[11]

Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. The immiscibility gaps in the plagioclase solid solutions are complex compared to the gap in the alkali feldspars. The play of colours visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae.

Barium feldspars

The barium feldspars are monoclinic and comprise the following:

Feldspars can form clay minerals through chemical weathering.[12]

Feldspar output in 2005.

Production and uses

Lunar ferrous anorthosite #60025 (plagioclase feldspar). Collected by Apollo 16 from the Lunar Highlands near Descartes Crater. This sample is currently on display at the National Museum of Natural History in Washington, D.C..

About 20 million tonnes of feldspar were produced in 2010, mostly by three countries: Italy (4.7 Mt), Turkey (4.5 Mt), and China (2 Mt).[13]

Feldspar is a common raw material used in glassmaking, ceramics, and to some extent as a filler and extender in paint, plastics, and rubber. In glassmaking, alumina from feldspar improves product hardness, durability, and resistance to chemical corrosion. In ceramics, the alkalis in feldspar (calcium oxide, potassium oxide, and sodium oxide) act as a flux, lowering the melting temperature of a mixture. Fluxes melt at an early stage in the firing process, forming a glassy matrix that bonds the other components of the system together. In the US, about 66% of feldspar is consumed in glassmaking, including glass containers and glass fiber. Ceramics (including electrical insulators, sanitaryware, pottery, tableware, and tile) and other uses, such as fillers, accounted for the remainder.[14]

In earth sciences and archaeology, feldspars are used for K-Ar dating, argon-argon dating, thermoluminescence dating, and optical dating.

In October 2012, the Mars Curiosity rover analyzed a rock that turned out to have a high feldspar content.[15]

Feldspar is the abrasive component in Bon Ami household cleaner.

See also


  • Bonewitz, Ronald Louis. (2005). Rock and Gem, New York: DK Publishing. ISBN 978-0-7566-3342-4

Further reading

  1. ^ "Feldspar". Gemology Online. Retrieved 8 November 2012. 
  2. ^ a b c d e f Feldspar. Industrial Minerals Association. Retrieved on July 18, 2007.
  3. ^ "Metamorphic Rocks." . Retrieved on July 18, 2007
  4. ^ Blatt, Harvey and Tracy, Robert J. (1996) Petrology, Freeman, 2nd ed., pp. 206–210 ISBN 0-7167-2438-3
  5. ^ "Weathering and Sedimentary Rocks." Retrieved on July 18, 2007.
  6. ^  
  7. ^ a b "The Mineral Orthoclase." Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
  8. ^ a b "Sanidine Feldspar." Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
  9. ^ "Microcline Feldspar." Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
  10. ^ Ralph, Jolyon and Chou, Ida. "Perthite." Retrieved on February 8, 2008.
  11. ^ Brown, Dwayne (October 30, 2012). "NASA Rover's First Soil Studies Help Fingerprint Martian Minerals".  
  12. ^ Nelson, Stephen A. (Fall 2008). "Weathering & Clay Minerals". Professor's lecture notes (EENS 211, Mineralogy). Tulane University. Retrieved 2008-11-13. 
  13. ^ Feldspar, USGS Mineral Commodity Summaries 2011
  14. ^ Apodaca, Lori E. Feldspar and nepheline syenite, USGS 2008 Minerals Yearbook
  15. ^ Nasa's Curiosity rover finds 'unusual rock' (12 October 2012) BBC News.


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