Feldspar
Feldspars (sometimes spelled felspars) are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium.[3] The most common members of the feldspar group are the plagioclase (sodium-calcium) feldspars and the alkali (potassium-sodium) feldspars.[4] Feldspars make up about 60% of the Earth's crust,[3] and 41% of the Earth's continental crust by weight.[5][6]
| Feldspar | |
|---|---|
 Feldspar crystal (18×21×8.5 cm) 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[1] |
| 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 | [2] |
Feldspars crystallize from magma as both intrusive and extrusive igneous rocks[7] and are also present in many types of metamorphic rock.[8] Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite.[9] Feldspars are also found in many types of sedimentary rocks.[10]