Feldspars
| Feldspar | |
|---|---|
Feldspar crystal (18×21×8.5 cm) from Jequitinhonha valley, Minas Gerais, Southeastern Brazil.
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| General | |
| Category | tectosilicate |
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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 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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