Feldspar is the name of a group of rock-forming
minerals which make up as much as 60% of the Earth's crust.Feldspar. July 18 2007.]
crystallize from magmain both intrusiveand extrusive igneousrocks, and they can also occur as compact minerals, as veins, and are also present in many types of metamorphic rock. ["Metamorphic Rocks." . Retrieved on July 18 2007.] Rock formed entirely of plagioclasefeldspar (see below) is known as anorthosite. ["Gem, Rock, and Mineral Postage Stamps Featuring Anorthosite." Retrieved on February 8 2008.] Feldspars are also found in many types of sedimentary rock. ["Weathering and Sedimentary Rocks." Retrieved on July 18 2007.]
Feldspar is derived from the German "Feld", field, and "Spat", a rock that does not contain
ore. "Feldspathic" refers to materials that contain feldspar. The alternative spelling, "felspar", has now largely fallen out of use. [Harper, Douglas. "Feldspar." Online Etymology Dictionary. Retrieved on February 8 2008.]
Industrial Minerals Association. Retrieved on
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, immiscibilityoccurs 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 are as follows:
orthoclase( monoclinic),"The Mineral Orthoclase." February 8 2008.] — KAlSi3O8
sanidine(monoclinic)"Sanidine Feldspar." February 8 2008.] —(K,Na)AlSi3O8
microcline( triclinic)"Microcline Feldspar." February 8 2008.] — KAlSi3O8
anorthoclase( triclinic) — (Na,K)AlSi3O8
Sanidine is stable at the highest temperatures, and microcline at the lowest. ] ]Amethyst Galleries, Inc. Retrieved on Amethyst Galleries, Inc. Retrieved on Amethyst Galleries, Inc. Retrieved on
Perthiteis a typical texture in alkali feldspar, due to exsolutionof 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. [Ralph, Jolyon & Ida. "Perthite." Retrieved on February 8 2008.] Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can only be seen using an electron microscope.
The plagioclase feldspars are
triclinic. The plagioclase series follows (with percent anorthite in parentheses):
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 solutionare complex compared to the gap in the alkali feldspars. The play of colors visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae.
The barium feldspars are monoclinic and comprise the following:
* Feldspar is a common raw material in the production of
ceramics and geopolymers.
* Feldspars are used for
thermoluminescence datingand optical datingin earth sciences and archaeology
* Feldspar is an ingredient in
Bon AmiUS brand household cleaner.
* Feldspar is often an anti-caking agent used in powdered forms of
In 2005, Italy was the top producer of feldspar with almost one-fifth world share followed by Turkey, China and Thailand, reports the
International Monetary Fund.
*Bonewitz, Ronald Louis. (2005). "Rock and Gem", New York: DK Publishing. ISBN 978-0-7566-3342-4
List of minerals
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