Feldspath

Feldspar
Feldspar crystal (18×21×8.5 cm) from Jequitinhonha valley, Minas Gerais, Southeastern Brazil.
General
Category	Tectosilicate
Formula (repeating unit)	KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8
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 (KAlSi₃O₈ – NaAlSi₃O₈ – CaAl₂Si₂O₈) are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight.

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.