The
mineral olivine is an
orthosilicate of
magnesium with the formula (Mg,Fe)
2SiO
4, 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