Feldspars (KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust.
Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic plagioclase feldspar (see below) is known as anorthosite. Feldspars are also found in many types of sedimentary rock.
Feldspar is derived from the German Feld, "field", and Spath, "a rock that does not contain ore". "Feldspathic" refers to materials that contain feldspar. The alternate spelling, felspar, has now largely fallen out of use.
This group of minerals consists of framework tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers:
Potassium-Feldspar (K-spar) endmember KAlSi3O8
Albite endmember NaAlSi3O8
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