Feldspar

Feldspar is the name of a group of rock-forming minerals which make up as much as 60% of the Earth's crust.Feldspar. [http://www.ima-na.org/about_industrial_minerals/feldspar.asp What is Feldspar?] Industrial Minerals Association. Retrieved on July 18 2007.]

Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, and they can also occur as compact minerals, as veins, and are also present in many types of metamorphic rock. ["Metamorphic Rocks." [http://seis.natsci.csulb.edu/bperry/ROCKS.htm Metamorphic Rocks Information] . Retrieved on July 18 2007.] Rock formed entirely of plagioclase feldspar (see below) is known as anorthosite. ["Gem, Rock, and Mineral Postage Stamps Featuring Anorthosite." [http://stampmin.home.att.net/anorthosite.htm Anorthosite] Retrieved on February 8 2008.] Feldspars are also found in many types of sedimentary rock. ["Weathering and Sedimentary Rocks." [http://geology.csupomona.edu/drjessey/class/Gsc101/Weathering.html Geology.] Retrieved on July 18 2007.]

Etymology

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." [http://www.etymonline.com/index.php?term=feldspar Feldspar] Online Etymology Dictionary. Retrieved on February 8 2008.]

Compositions

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

K-feldspar endmember
KAlSi₃O₈

Albite endmember
NaAlSi₃O₈

Anorthite endmember
CaAl₂Si₂O₈

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.

The alkali feldspars are as follows:
* orthoclase (monoclinic),"The Mineral Orthoclase." [http://www.galleries.com/minerals/silicate/orthocla/orthocla.htm Feldspar] Amethyst Galleries, Inc. Retrieved on February 8 2008.] — KAlSi₃O₈
* sanidine (monoclinic)"Sanidine Feldspar." [http://www.galleries.com/minerals/silicate/sanidine/sanidine.htm Feldspar] Amethyst Galleries, Inc. Retrieved on February 8 2008.] —(K,Na)AlSi₃O₈
*microcline (triclinic)"Microcline Feldspar." [http://www.galleries.com/minerals/silicate/microcli/microcli.htm Feldspar] Amethyst Galleries, Inc. Retrieved on February 8 2008.] — 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. [Ralph, Jolyon & Ida. "Perthite." [http://www.mindat.org/min-6683.html Perthite Profile on mindat.org] 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 solution are 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:
* celsian — BaAlSi₃O₈
* hyalophane — (K,Na,Ba)(Al,Si)₄O₈

Uses

* Feldspar is a common raw material in the production of ceramics and geopolymers.
* Feldspars are used for thermoluminescence dating and optical dating in earth sciences and archaeology
* Feldspar is an ingredient in Bon Ami US brand household cleaner.
* Feldspar is often an anti-caking agent used in powdered forms of non-dairy creamer.

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.

References

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

ee also

*List of minerals