Chemical Group: Silicate

Olivine is a ferro-magnesian mineral that shows a complete compositional variation between forsterite (magnesium-rich) and fayalite (iron-rich). Olivine is light green in colour and glassy. The internal arrangment of the atoms is constructed of isolated silica tetrahedra (SiO4) held together ionically by iron and magnesium ions. With no planes of weakness there is no cleavage, and because the crystals grow outward in all directions from a tetrahedral seed they are described as granular.

Olivine is a rock-forming mineral, and occurs in igneous rocks with low silica, such as a gabbro, basalt, or dolerite. This is a relatively rare mineral in most igneous rocks, or one that is hard to see without a microscope. Large accumulations form the igneous rock dunite or, when mixed with pyroxene, peridotite. Olivine is rare as a detrital mineral - on Earth* - owing to its low stability in sedimentary environments. Forsterite can occur in metamorphic marble. Fayalite is dark brown or black; it is uncommon but can occur in granite pegmatites.

*Olivine appears to be stable as a detrital mineral under the cold, dry conditions at the surface of Mars.

Orthorhombic Olive green 6.5 - 7 3.27 - 4.20 Vitreous