Ferrous magnesium orthosilicate, (Fe,Mg)SiO₄

Ferrous magnesium orthosilicate, (Fe,Mg)SiO₄ or Fe₂SiO₄ + xMg₂SiO₄, where x ranges from 3 to 12, occurs as green rhombic crystals, usually small in size. It is present in slag, being the silicate of highest melting-point, namely, about 1400° C. Natural olivines usually melt between 1363° and 1378° C.

The refractive index of olivine progressively increases with the iron content, and may be used as a criterion of the chemical composition of rock sections.

Olivine readily decomposes upon exposure, yielding serpentine, Mg₃Si₂O₇.2H₂O.