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The mineral olivine is an orthosilicate of magnesium 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 specific gravity 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.

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.

See also: List of minerals

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