forsterit

Forsterite is the magnesium-rich end-member of the olivine group, a magnesium silicate with the chemical formula Mg2SiO4. It forms a continuous solid solution with fayalite, the iron end-member Fe2SiO4, and is commonly described by its Fo value, indicating the molar magnesium content in olivine grains.

In terms of structure and appearance, forsterite crystallizes in the orthorhombic system. Crystals are typically prismatic and may be olive-green to colorless, with impurities producing yellowish to brown tints. It has a relatively high hardness on the Mohs scale, about 6.5 to 7, and a conchoidal to uneven fracture. Cleavage is poor or indistinct.

Forsterite occurs mainly in ultramafic igneous rocks such as dunite and peridotite, and is also found in basaltic magmas as phenocrysts. It forms at high temperatures and low silica activity, and is abundant in the Earth's upper mantle, where it persists to form mantle rocks and xenoliths. It can also be present in meteorites and certain metamorphic rocks, often accompanied by pyroxene, spinel, chromite, and magnetite.

The mineral has limited direct commercial use, but olivine-rich rocks are important as sources of magnesium and as refractory materials. Gem-quality olivine, commonly referred to as peridot, is a gemstone that typically represents magnesium-rich compositions close to forsterite, though natural samples can contain iron that shifts color and stability. In petrology and mineralogy, forsterite serves as a reference end-member for discussing mantle composition and magmatic differentiation.