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The mineral olivine is an orthosilicate of magnesia with the formula (Mg,Fe)2SiO4, in which the ratio of magnesium and iron is found to vary between the 2 endmembers of forsterite (Mg-rich) and fayalite (Fe-rich). Olivine crystallizes in the orthorhombic system in somewhat flattened forms but may occur massive or granular. It has a conchoidal fracture and is rather brittle. The hardness of olivine is 6.5-7, its relative density is 3.27-3.37 and it has a vitreous lustre. It is usually colored olive-green (hence the name), though it may be reddish from the oxidation of iron. It is transparent to translucent. Olivine occurs in both igneous rocks as a primary mineral and in certain metamorphic rocks, and has also been discovered in meteorites as well as on Mars.
Olivine forms from magma that is rich in magnesia and low in silica, forming such rocks as gabbro, norite, peridotite and basalt. The metamorphosis of impure dolomite or other sedimentary rock with high magnesia and low silica content also seems to produce olivine.
Transparent olivine is sometimes used as a gemstone, often called peridot, the French word for olivine. It is also called chrysolite from the Greek words for gold and stone.
Traces of olivine were observed by the Mars Odyssey orbiter on the surface of Mars, suggesting the possibility of liquid water in the past.
See also: List of minerals, Bowen's reaction series
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