Feldspar

Lunar Ferroan Anorthosite #60025 (Plagioclase Feldspar). Collected by Apollo 16 from the Lunar Highlands near Descartes Crater. This sample is currently on display at the National Museum of Natural History in Washington, DC. United States (Unknown scale)

Feldspar is the name of a group of rock-forming minerals which make up as much as 60% of the Earth's crust.^[1]

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.^[2] Rock formed entirely of plagioclase feldspar (see below) is known as anorthosite.^[3] Feldspars are also found in many types of sedimentary rock.^[4]

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.^[5]

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^[1]

Albite endmember NaAlSi₃O₈^[1]

Anorthite endmember CaAl₂Si₂O₈^[1]

Solid solutions between K-feldspar and albite are called alkali feldspar.^[1] Solid solutions between albite and anorthite are called plagioclase^[1], 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 potassium is replaced by barium.

The alkali feldspars are as follows:

• orthoclase (monoclinic),^[6] — KAlSi₃O₈
  • sanidine (monoclinic)^[7] —(K,Na)AlSi₃O₈
    • microcline (triclinic)^[8] — KAlSi₃O₈
      • anorthoclase (triclinic) — (Na,K)AlSi₃O₈

      Sanidine is stable at the highest temperatures, and microcline at the lowest.^[7][6] 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 are coarse enough to be visible to the naked eye.^[9]

      
      

      

      

      Labradorite

      The pagioclase 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

      Image:2005feldspar.PNG

      Feldspar output in 2005. Click the image for the details.
      • Feldspar is a common raw material in the production of ceramics.
      • 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

      Image:Question book-3.svg

      This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (April 2007)

      1. ^ ^{a b c d e f} Feldspar. What is Feldspar? Industrial Minerals Association. Retrieved on July 18 2007.
      2. ^ "Metamorphic Rocks." Metamorphic Rocks Information. Retrieved on July 18 2007.
      3. ^ "Gem, Rock, and Mineral Postage Stamps Featuring Anorthosite." Anorthosite Retrieved on February 8 2008.
      4. ^ "Weathering and Sedimentary Rocks." Geology. Retrieved on July 18 2007.
      5. ^ Harper, Douglas. "Feldspar." Feldspar Online Etymology Dictionary. Retrieved on February 8 2008.
      6. ^ ^{a b} "The Mineral Orthoclase." Feldspar Amethyst Galleries, Inc. Retrieved on February 8 2008.
      7. ^ ^{a b} "Sanidine Feldspar." Feldspar Amethyst Galleries, Inc. Retrieved on February 8 2008.
      8. ^ "Microcline Feldspar." Feldspar Amethyst Galleries, Inc. Retrieved on February 8 2008.
      9. ^ Ralph, Jolyon & Ida. "Perthite." Perthite Profile on mindat.org Retrieved on February 8 2008.

      
      

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

      See also

      

      Wikimedia Commons has media related to:

      Category:Feldspar
      • List of mineralsar:مجموعة الفلدسبار

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