The olivine group is made up of eight minerals, including forsterite (magnesium silicate, MgSiO4) and fayalite (iron silicate, (FeSiO4). These minerals are end members of a solid-solution series based on the mutual substitution of iron and magnesium.
Olivine crystals are formed at depths of at least 25 miles below the earth’s surface, in the high pressure zone of the Earth’s upper mantle. Volcanic eruptions bring the magma to the surface and these magmas solidify into basalt and basaltic pumice.
For example, Kilauea has a magma chamber that is between 3 to 6 miles beneath the surface but draws magma from much deeper where olivine crystals are present.