wüstite

Wüstite is iron(II) oxide, FeO, best described as the non-stoichiometric oxide Fe1−xO. The value of x depends on temperature and the oxygen fugacity of the environment and typically ranges up to about 0.15, giving compositions from FeO toward Fe0.85O. The crystal structure is rock-salt (face-centered cubic) with iron vacancies that maintain charge balance, and small amounts of Fe3+ can occur to compensate defects.

Because of its non-stoichiometry, wüstite is not a fixed compound but a defective oxide. As temperature or oxidation conditions change, the vacancy concentration and the ratio of Fe2+ to Fe3+ vary, leading to a range of compositions rather than a single formula. This defect chemistry also affects its electrical conductivity and color, and makes precise characterization challenging.

In practice, wüstite is stable only under relatively reducing conditions and at high temperatures. It forms during the reduction of iron oxides and is an intermediate phase in steelmaking and in high-temperature oxide systems. In nature, pure wüstite is extremely rare at Earth’s surface because it readily oxidizes to other iron oxides; natural occurrences are limited to specific high-temperature or extraterrestrial settings, such as inclusions in meteorites or high-temperature oxide assemblages.

Phase relations in the iron–oxygen system place wüstite between metallic iron (at very low oxygen) and the oxides magnetite and hematite (at higher oxygen). Its presence is a key feature in understanding oxidation, reduction, and corrosion processes in iron-containing materials, as well as the thermodynamics of the Fe–O system.