Olivine

The mineral olivine (/ˈɒl.ɪˌvn/) is a magnesium iron silicate with the chemical formula (Mg,Fe)2SiO4. It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle,[9] it is a common mineral in Earth's subsurface, but weathers quickly on the surface. For this reason, olivine has been proposed as a good candidate for accelerated weathering to sequester carbon dioxide from the Earth's oceans and atmosphere, as part of climate change mitigation. Olivine also has many other historical uses, such as the gemstone peridot (or chrysolite), as well as industrial applications like metalworking processes.

Olivine in cross-polarized light
Olivine
General
CategoryNesosilicate
Olivine group
Olivine series
Formula
(repeating unit)
(Mg,Fe)2SiO4
IMA symbolOl[1]
Strunz classification9.AC.05
Crystal systemOrthorhombic
Space groupPbnm (no. 62)
Identification
ColorYellow to yellow-green
Crystal habitMassive to granular
CleavagePoor
FractureConchoidal
Tenacitybrittle
Mohs scale hardness6.5–7
LusterVitreous
Streakcolorless or white
DiaphaneityTransparent to translucent
Specific gravity3.2–4.5[2][3][4][5]
Optical propertiesBiaxial (+)
Refractive indexnα = 1.630–1.650
nβ = 1.650–1.670
nγ = 1.670–1.690
Birefringenceδ = 0.040
References[6][7][8]

The ratio of magnesium to iron varies between the two endmembers of the solid solution series: forsterite (Mg-endmember: Mg
2
SiO
4
) and fayalite (Fe-endmember: Fe
2
SiO
4
). Compositions of olivine are commonly expressed as molar percentages of forsterite (Fo) and fayalite (Fa) (e.g., Fo70Fa30). Forsterite's melting temperature is unusually high at atmospheric pressure, almost 1,900 °C (3,450 °F), while fayalite's is much lower – about 1,200 °C (2,190 °F). Melting temperature varies smoothly between the two endmembers, as do other properties. Olivine incorporates only minor amounts of elements other than oxygen (O), silicon (Si), magnesium (Mg) and iron (Fe). Manganese (Mn) and nickel (Ni) commonly are the additional elements present in highest concentrations.

Olivine gives its name to the group of minerals with a related structure (the olivine group) – which includes tephroite (Mn2SiO4), monticellite (CaMgSiO4), larnite (Ca2SiO4) and kirschsteinite (CaFeSiO4) (commonly also spelled kirschteinite[10]).

Olivine's crystal structure incorporates aspects of the orthorhombic P Bravais lattice, which arise from each silica (SiO4) unit being joined by metal divalent cations with each oxygen in SiO4 bound to three metal ions. It has a spinel-like structure similar to magnetite but uses one quadrivalent and two divalent cations M22+ M4+O4 instead of two trivalent and one divalent cations.[11]


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