Feo

Iron(II) oxide, FeO, is an inorganic compound that occurs naturally mainly as the mineral wüstite. In the solid state it adopts the rock-salt crystal structure and is often non-stoichiometric, written as Fe1−xO. Typical compositions range from about Fe0.95O to Fe0.87O, with iron vacancies and a small amount of Fe3+ helping to balance charge. As a result, FeO is described as a nonstoichiometric oxide and a defect-rich material.

FeO is stable at high temperatures and under reducing conditions. Industrially, it can be prepared by reducing iron oxides with carbon at elevated temperatures. In nature, the mineral wüstite is relatively rare and is found in specific high-temperature, low-oxygen environments such as certain meteorites and high-temperature zones within planetary crusts. Its presence in broader geological settings is typically transient due to its tendency to oxidize.

In air or more oxidizing atmospheres, FeO is not a long-term stable phase. It readily oxidizes to magnetite (Fe3O4) and then to ferric oxides such as hematite (Fe2O3) as the oxygen fugacity increases or the temperature changes. FeO also exhibits a temperature-dependent magnetic behavior, being antiferromagnetic below about 198 kelvin and paramagnetic at higher temperatures.

Applications of FeO arise mainly from its role as a chemical intermediate and its historical use as a pigment (ferrous oxide). In metallurgy, FeO and related iron oxides help control the oxygen potential during processing and can serve as precursors to other iron oxides and ferrites. Safety considerations for finely divided FeO align with typical inorganic oxides: generally low toxicity but possible respiratory irritation if inhaled.