Feldspar

Feldspar
Feldspar crystal (18×21×8.5 cm) from Jequitinhonha valley, Minas Gerais, southeastern Brazil
General
Category Tectosilicate
Formula
(repeating unit)		 KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8
Crystal system Triclinic or monoclinic
Identification
Color pink, white, gray, brown
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 (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight.[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 rocks.[5]

Etymology

The name feldspar derives from the German Feldspat, a compound of the words Feld, "field", and Spat, "a rock that does not contain ore".[6] The change from Spat to -spar was influenced by the English word spar,[7] meaning a non-opaque mineral with good cleavage.[8] Feldspathic refers to materials that contain feldspar. The alternate spelling, felspar, has largely fallen out of use.

Compositions

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

Solid solutions between K-feldspar and albite are called "alkali feldspar".[9] Solid solutions between albite and anorthite are called "plagioclase",[9] 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.

Alkali feldspars

Alkali feldspars are grouped into two types, those containing potassium in combination with sodium, aluminum, or silicon, and those where potassium is replaced by barium. The first of these include:

Sanidine is stable at the highest temperatures, and microcline at the lowest.[10][11] 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.[13] Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.

Barium feldspars

Barium feldspars are also considered alkali feldspars. Barium feldspars form as the result of the substitution of barium for potassium in the mineral structure. The barium feldspars are monoclinic and include the following:

Plagioclase feldspars

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

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. The specific gravity in the plagioclase series increases from albite (2.62 g/cm3) to anorthite (2.72–2.75 g/cm3).

Weathering

Chemical weathering of feldspars results in the formation of clay minerals[16] such as illite and kaolinite.

Production and uses

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).[17]

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.[18]

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

In October 2012, the Mars Curiosity rover analyzed a rock that turned out to have a high feldspar content.[19]

Images

See also

References

  •  This article incorporates public domain material from the United States Geological Survey document: "Feldspar and nepheline syenite" (PDF).
  1. ↑ "Feldspar". Gemology Online. Retrieved 8 November 2012.
  2. ↑ Anderson, Robert S.; Anderson, Suzanne P. (2010). Geomorphology: The Mechanics and Chemistry of Landscapes. Cambridge University Press. p. 187.
  3. ↑ "Metamorphic Rocks." Metamorphic Rocks Information Archived 2007-07-01 at the Wayback Machine.. 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." Geology. Archived 2007-07-21 at WebCite Retrieved on July 18, 2007.
  6. ↑ Harper, Douglas. "feldspar". Online Etymology Dictionary. Retrieved 2008-02-08.
  7. ↑ Harper, Douglas. "feldspar". Online Etymology Dictionary. Retrieved 2008-02-08.
  8. ↑ "spar". Oxford English Dictionary. Oxford Dictionaries. Retrieved 13 January 2018.
  9. 1 2 3 4 5 Feldspar. What is Feldspar? Industrial Minerals Association. Retrieved on July 18, 2007.
  10. 1 2 "The Mineral Orthoclase". Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
  11. 1 2 "Sanidine Feldspar". Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
  12. ↑ "Microcline Feldspar". Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
  13. ↑ Ralph, Jolyon and Chou, Ida. "Perthite". Perthite Profile on mindat.org. Retrieved on February 8, 2008.
  14. ↑ Celsian–orthoclase series on Mindat.org.
  15. ↑ Celsian–hyalophane series on Mindat.org.
  16. ↑ Nelson, Stephen A. (Fall 2008). "Weathering & Clay Minerals". Professor's lecture notes (EENS 211, Mineralogy). Tulane University. Retrieved 2008-11-13.
  17. ↑ Feldspar, USGS Mineral Commodity Summaries 2011
  18. ↑ Apodaca, Lori E. Feldspar and nepheline syenite, USGS 2008 Minerals Yearbook
  19. ↑ Nasa's Curiosity rover finds 'unusual rock'. (12 October 2012) BBC News.
  20. ↑ Brown, Dwayne (October 30, 2012). "NASA Rover's First Soil Studies Help Fingerprint Martian Minerals". NASA. Retrieved October 31, 2012.

Further reading

  • Bonewitz, Ronald Louis (2005). Rock and Gem. New York: DK Publishing. ISBN 978-0-7566-3342-4.
  • Media related to Feldspar at Wikimedia Commons
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