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Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic plagioclase feldspar (see below) is known as anorthosite. Feldspars are also found in many types of sedimentary rock.
The name feldspar derives from the German words Feld, "field", and Spath, "a rock that does not contain ore." "Feldspathic" refers to materials that contain feldspar. The alternate spelling, felspar, has largely fallen out of use.
Solid solutions between K-feldspar and albite are called alkali feldspar. Solid solutions between albite and anorthite are called plagioclase, 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:
- orthoclase (monoclinic), — KAlSi3O8
- sanidine (monoclinic) —(K,Na)AlSi3O8
- microcline (triclinic) — KAlSi3O8
- anorthoclase (triclinic) — (Na,K)AlSi3O8
Sanidine is stable at the highest temperatures, and microcline at the lowest. 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. Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.
Plagioclase feldspars 
- albite (0 to 10) — NaAlSi3O8
- oligoclase (10 to 30) — (Na,Ca)(Al,Si)AlSi2O8
- andesine (30 to 50) — NaAlSi3O8 — CaAl2Si2O8
- labradorite (50 to 70) — (Ca,Na)Al(Al,Si)Si2O8
- bytownite (70 to 90) — (NaSi,CaAl)AlSi2O8
- anorthite (90 to 100) — CaAl2Si2O8
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:
Production and uses 
About 20 million tonnes of feldspar was produced in 2010, mostly by three countries: Italy (4.7 Mt), Turkey (4.5 Mt), and China (2 Mt).
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. Pottery (including electrical insulators, sanitaryware, tableware, and tile) and other uses, such as fillers, accounted for the remainder.
See also 
|Wikimedia Commons has media related to: Feldspar|
- "Feldspar". Gemology Online. Retrieved 8 November 2012.
- Feldspar. Industrial Minerals Association. Retrieved on July 18, 2007.
- "Metamorphic Rocks." . Retrieved on July 18, 2007
- Blatt, Harvey and Robert J. Tracy, Petrology, Freeman, 2nd ed., 1996, pp. 206–210 ISBN 0-7167-2438-3
- "Weathering and Sedimentary Rocks." Retrieved on July 18, 2007.
- Harper, Douglas. "feldspar". Online Etymology Dictionary. Retrieved 2008-02-08.
- "The Mineral Orthoclase." Amethyst Galleries, Inc. Retrieved on February 8, 2008.
- "Sanidine Feldspar." Amethyst Galleries, Inc. Retrieved on February 8, 2008.
- "Microcline Feldspar." Amethyst Galleries, Inc. Retrieved on February 8, 2008.
- Ralph, Jolyon & Ida. "Perthite." Retrieved on February 8, 2008.
- Brown, Dwayne (October 30, 2012). "NASA Rover's First Soil Studies Help Fingerprint Martian Minerals". NASA. Retrieved October 31, 2012.
- Nelson, Stephen A. (Fall 2008). "Weathering & Clay Minerals". Professor's lecture notes (EENS 211, Mineralogy). Tulane University. Retrieved 2008-11-13.
- Feldspar, USGS Mineral Commodity Summaries 2011
- Lori E Apodaca Feldspar and nepheline syenite, USGS 2008 Minerals Yearbook
- Nasa's Curiosity rover finds 'unusual rock' (12 October 2012) BBC News.
Further reading 
- Bonewitz, Ronald Louis. (2005). Rock and Gem, New York: DK Publishing. ISBN 978-0-7566-3342-4