Olivine group
| Olivine | |
|---|---|
 |
|
| General | |
| Category |
Nesosilicate Olivine group Olivine series |
|
Formula (repeating unit) |
(Mg, Fe)2SiO4 |
| Strunz classification | 9.AC.05 |
| Crystal system | Orthorhombic |
| Identification | |
| Color | Yellow to yellow-green |
| Crystal habit | Massive to granular |
| Cleavage | Poor |
| Fracture | Conchoidal – brittle |
| Mohs scale hardness | 6.5–7 |
| Luster | Vitreous |
| Streak | None |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.2–4.5 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.630–1.650 nβ = 1.650–1.670 nγ = 1.670–1.690 |
| Birefringence | δ = 0.040 |
| References | |
The mineral olivine ( /ˈɒlɪˌviːn/) is a magnesium iron silicate with the formula (Mg2+, Fe2+)2SiO4. Thus it is a type of nesosilicate or orthosilicate. It is a common mineral in the Earth's subsurface but weathers quickly on the surface.
The ratio of magnesium and iron varies between the two endmembers of the solid solution series: forsterite (Mg-endmember: Mg2SiO4) and fayalite (Fe-endmember: Fe2SiO4). Compositions of olivine are commonly expressed as molar percentages of forsterite (Fo) and fayalite (Fa) (e.g., Fo70Fa30). Forsterite has an unusually high melting temperature at atmospheric pressure, almost 1,900 °C (3,450 °F), but the melting temperature of fayalite is much lower (about 1,200 °C [2,190 °F]). The melting temperature varies smoothly between the two endmembers, as do other properties. Olivine incorporates only minor amounts of elements other than oxygen, silicon, magnesium and iron. Manganese and nickel commonly are the additional elements present in highest concentrations.
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