Mg2SiO4

Mg2SiO4 is magnesium orthosilicate, the magnesium endmember of the olivine group. In geology, it is commonly referred to as forsterite. It forms a continuous solid solution series with fayalite (Fe2SiO4), from which varying iron content yields the olivine minerals found in many igneous and metamorphic rocks.

Crystal and mineralogy: Forsterite crystallizes in the orthorhombic system. The crystal structure consists of isolated SiO4 tetrahedra linked by Mg2+ in octahedral coordination, creating the characteristic olivine framework. The mineral is typically colorless to pale olive-green, transparent to translucent, with a vitreous luster. It has a Mohs hardness of about 6.5–7 and a specific gravity near 3.2–3.4. Cleavage is weak or absent; fractures are conchoidal.

Occurrence and formation: Forsterite is a major constituent of ultramafic rocks such as peridotites and dunites and occurs in many ultramafic-hosted magmas. It can crystallize from high-temperature mafic to ultramafic magmas and is common in the Earth's upper mantle. It is also found in meteorites and, rarely, in some high-temperature metamorphic rocks. In the presence of water, olivine readily alters to serpentine-group minerals and brucite.

Uses and significance: The mineral is an important source of magnesium oxide (MgO) and is used as a refractory material in high-temperature industrial applications and as a ceramic raw material. Forsterite-rich rocks are diagnostic of mantle-derived processes.