Feldspar

For other uses, see Feldspar (disambiguation).
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 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 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". The change from Spat to -spar was influenced by the English word "spar", a synonym for "mineral".[6] 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:

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.

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.

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.

Weathering

Chemical weathering of feldspars results in the formation of clay minerals.[13]

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).[14]

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.[15]

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.[16]

  1. ^ Brown, Dwayne (October 30, 2012). "NASA Rover's First Soil Studies Help Fingerprint Martian Minerals". NASA. Retrieved October 31, 2012. 

See also

References

  1. ↑ "Feldspar". Gemology Online. Retrieved 8 November 2012.
  2. 1 2 3 4 5 6 Feldspar. What is Feldspar? Industrial Minerals Association. Retrieved on July 18, 2007.
  3. ↑ "Metamorphic Rocks." Metamorphic Rocks Information. 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. Retrieved on July 18, 2007.
  6. ↑ Harper, Douglas. "feldspar". Online Etymology Dictionary. Retrieved 2008-02-08.
  7. 1 2 "The Mineral Orthoclase". Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
  8. 1 2 "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". Perthite Profile on mindat.org Retrieved on February 8, 2008.
  11. ↑ Celsian–orthoclase series on Mindat.org
  12. ↑ Celsian–hyalophane series on Mindat.org
  13. ↑ Nelson, Stephen A. (Fall 2008). "Weathering & Clay Minerals". Professor's lecture notes (EENS 211, Mineralogy). Tulane University. Retrieved 2008-11-13.
  14. ↑ Feldspar, USGS Mineral Commodity Summaries 2011
  15. ↑ Apodaca, Lori E. Feldspar and nepheline syenite, USGS 2008 Minerals Yearbook
  16. ↑ Nasa's Curiosity rover finds 'unusual rock'. (12 October 2012) BBC News.

Further reading

External links

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