Fayalite

Fayalite is an iron-rich silicate mineral in the olivine group, with the chemical formula Fe2SiO4. It represents the iron end-member of the olivine solid solution series, with forsterite (Mg2SiO4) serving as the magnesium end-member. Fayalite crystallizes in the orthorhombic system and commonly occurs in high-temperature igneous rocks as a primary phase or within olivine-rich rocks.

In terrestrial rocks, fayalite is less common than forsterite but can occur in mafic and ultramafic rocks such as basalt, gabbro, and peridotite, especially where iron is abundant and the environment is relatively reducing. Fayalite is also reported in some meteorites and in high-temperature metamorphic rocks where olivine has become iron-enriched.

Fayalite typically appears brown to brownish-black in hand specimens and is translucent to opaque in crystals. It has a hardness of about 6.5–7 on the Mohs scale and a specific gravity around 4.3–4.5. It lacks true cleavage and generally exhibits a conchoidal to uneven fracture. In thin section, olivine minerals, including fayalite, show strong anisotropy and characteristic relief; the iron content gives fayalite a brownish coloration in transmitted light.

Geologically, the fayalite content of olivine is used as a redox and geothermobarometric indicator. Stability relations between fayalite-bearing olivine, magnetite, and quartz help constrain the oxidation state and temperature–pressure conditions during rock formation, making fayalite a useful marker in geologic studies of magmatic and metamorphic processes. See also forsterite; olivine.