Quiz: Remember the definition of mal de mer?
An olive-green to brownish-green orthorhombic mineral. Olivine is a common mineral in the igneous rocks, such as basalt and gabbro, that make up most of the Earth's crust beneath the oceans. Chemical formula: (Mg,Fe)2SiO4. ◇ Olivine in which the mafic component consists entirely of magnesium is called forsterite. Chemical formula: Mg2SiO4. ◇ Olivine in which the mafic component consists entirely of iron is called fayalite. Chemical formula: Fe2SiO4.