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Feldspar

2007 Schools Wikipedia Selection. Related subjects: Mineralogy

   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.
   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.

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

   Feldspars crystallize from magma in both intrusive and extrusive rocks,
   and they can also occur as compact minerals, as veins, and are also
   present in many types of metamorphic rock. Rock formed entirely of
   plagioclase feldspar (see below) is known as anorthosite. Feldspars are
   also found in many types of sedimentary rock.

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.

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 KAlSi[3]O[8]

   Albite endmember NaAlSi[3]O[8]

   Anorthite endmember CaAl[2[Si[2]O[8]]]

   Solid solutions between K-feldspar and albite are called alkali
   feldspar. Solid solutions between albite and anorthite are called
   plagioclase. 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.

   Sanidine ( monoclinic), orthoclase, and microcline ( triclinic) refer
   to polymorphs of K-feldspar. 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 are
   coarse enough to be visible to the naked eye.

   Compositions of the plagioclase series have been labeled as follows
   (percent anorthite in parentheses):
     *
          + albite (0 to 10)
          + oligoclase (10 to 30)
          + andesine (30 to 50)
          + labradorite (50 to 70)
          + bytownite (70 to 90)
          + anorthite (90 to 100)

   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 ( see image).

Uses

     * 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 brand household cleaner.

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