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Olivine group

Olivine
Olivine-gem7-10a.jpg
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 ( /ˈɒlɪˌvn/) 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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