Olivine is a silicate mineral with the general composition (Mg,Fe)2SiO4. Having a very high crystallization temperature, it is formed early in the Bowen reaction series. It has no cleavage planes and is black to dark green in color. The absence of cleavage planes contributes to a glassy appearance, and any fractures will be conchoidal rather than smooth and planar. The dark color is associated with the high content of iron and magnesium, making olivine an example of a "ferromagnesian" mineral.
Though not abundant in continental rocks, olivine is yet thought to be a major component of the upper mantle of the Earth. Part of the basis for this presumption is the fact that it is one of the first minerals to crystallize from molten magma.
In Olivine the Fe2+ and Mg2+ freely substitute for each other. The Fe2+ and Mg2+ ions have almost the same size, 0.074 nm and 0.066 nm respectively, so the substitution makes very little structural change. The chemical formula (Mg,Fe)2SiO4 implies any mixture of iron and magnesium. It is said to be a "family" of minerals rather than one which has a definite composition.
Lutgens & Tarbuck