Feldspar


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


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Feldspar — Feld spar , Feldspath Feld spath , n. [G. feldspath; feld field + spath spar.] (Min.) A name given to a group of minerals, closely related in crystalline form, and all silicates of alumina with either potash, soda, lime, or, in one case, baryta.… …   The Collaborative International Dictionary of English

  • feldspar — 1785, earlier feldspath (1757), from Ger. Feldspath (Mod.Ger. Feldspat), from Feld field (see FIELD (Cf. field) (n.)) + spath spar, non metallic mineral, gypsum (see SPAR (Cf. spar) (n.2)); spelling influenced by English spar …   Etymology dictionary

  • feldspar — (also felspar) ► NOUN ▪ a pale rock forming silicate mineral. ORIGIN German Feldspat field spar …   English terms dictionary

  • feldspar — [feld′spär΄] n. [altered < Ger feldspath (now feldspat) < feld, FIELD + spath, spar < IE base * spe , long, flat board > SPOON, SPADE1] any of several light colored, hard, glassy, monoclinic or triclinic minerals made up of aluminum… …   English World dictionary

  • feldspar — /feld spahr , fel /, n. any of a group of minerals, principally aluminosilicates of potassium, sodium, and calcium, characterized by two cleavages at nearly right angles: one of the most important constituents of igneous rocks. Also, felspar.… …   Universalium

  • feldspar — noun Etymology: modification of obsolete German Feldspath (now Feldspat), from German Feld field + obsolete German Spath (now Spat) spar Date: 1772 any of a group of crystalline minerals that consist of aluminum silicates with either potassium,… …   New Collegiate Dictionary

  • feldspar — noun Any of a large group of rock forming minerals that, together, make up about 60% of the earths outer crust. The feldspars are all aluminum silicates of the alkali metals sodium, potassium, calcium and barium. Feldspars are the principal… …   Wiktionary

  • feldspar —   the generic name for a group of minerals made up of aluminium silicate along with calcium, potassium or sodium. A component of granite …   Geography glossary

  • feldspar — feld|spar [ˈfeldspa: US a:r] n [U] [Date: 1700 1800; : German; Origin: feldspath, from feld field + spath type of mineral] a type of grey or white mineral …   Dictionary of contemporary English

  • feldspar — feld|spar [ fel,spar ] noun uncount a type of hard pale stone …   Usage of the words and phrases in modern English


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.