forsterite

Forsterite is the magnesium-rich end-member of the olivine group, a silicate mineral with the chemical composition Mg2SiO4. It forms a solid solution with fayalite (Fe2SiO4) across the (Mg,Fe)2SiO4 series, and compositions are often expressed as Fo# (molar Mg/(Mg+Fe)). Forsterite crystallizes in the orthorhombic system and is commonly found in ultramafic rocks.

Geologically, forsterite occurs most often in the Earth's upper mantle and in rocks derived from mantle material, such as peridotite and dunite. It can also appear in basaltic rocks, meteorites, and lunar samples, or as grains in mantle-derived xenoliths brought to the surface by volcanism. In hand specimen it is typically colorless to pale olive green, with color deepening as iron content increases, and it may exhibit pleochroism and high birefringence under polarized light.

Formation and stability of forsterite are controlled by temperature, pressure, and the iron-magnesium ratio of the rock. It crystallizes from high-temperature magmas and remains stable at high temperatures in the mantle, while evolving to fayalite-rich compositions at lower temperatures or higher iron contents. As a key mantle mineral, forsterite provides information about the composition and evolution of mantle rocks.

Physical properties commonly cited include a hardness around 6.5–7 on the Mohs scale and a specific gravity of about 3.2–3.4, varying with composition. Industrially, forsterite is valued for its high melting point and chemical stability as a refractory material, used in furnace linings and specialized ceramics.

Forsterite thus serves as both a primary mantle mineral and a useful industrial raw material, marking the magnesium end-member of the olivine series.