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Feldspars

Feldspar
Feldspar-Group-291254.jpg
Feldspar crystal (18×21×8.5 cm) from Jequitinhonha valley, Minas Gerais, Southeastern Brazil.
General
Category tectosilicate
Formula
(repeating unit)
KAlSi3O8NaAlSi3O8CaAl2Si2O8
Crystal system Triclinic or monoclinic
Identification
Color pink, white, gray, brown
Cleavage two or three
Fracture along cleavage planes
Mohs scale hardness 6.0–6.5
Luster Vitreous
Streak white
Diaphaneity opaque
Specific gravity 2.55–2.76
Density 2.56
Refractive index 1.518–1.526
Birefringence first order
Pleochroism none
Other characteristics exsolution lamellae common
References

Feldspars (KAlSi3O8NaAlSi3O8CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up about 40% of the Earth's continental crust.

Feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks 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 rocks.

The name feldspar derives from the German Feldspat, a compound of the words Feld, "field", and Spat, "a rock that does not contain ore". The change from Spat to -spar was influenced by the English word "spar", a synonym for "mineral".Feldspathic refers to materials that contain feldspar. The alternate spelling, felspar, has largely fallen out of use.


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