Feldspar
Feldspar (/ˈfɛl(d)ˌspɑːr/ FEL(D)-spar; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the plagioclase (sodium-calcium) feldspars and the alkali (potassium-sodium) feldspars. Feldspars make up about 60% of the Earth's crust, and 41% of the Earth's continental crust by weight.
| Feldspar | |
|---|---|
Feldspar crystal 18 cm × 21 cm × 8.5 cm (7.1 in × 8.3 in × 3.3 in) from Jequitinhonha valley, Minas Gerais, southeastern Brazil | |
| General | |
| Category | Tectosilicate |
| Formula (repeating unit) | KAlSi 3O 8 – NaAlSi 3O 8 – CaAl 2Si 2O 8 |
| IMA symbol | Fsp |
| Crystal system | Triclinic or monoclinic |
| Identification | |
| Color | pink, white, gray, brown, blue |
| 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 crystallize from magma as both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite. Feldspars are also found in many types of sedimentary rocks.
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