Iron(II) oxide, also known by its former name ferrous oxide or informally as iron monoxide, is one of the iron oxides. It is a black-colored powder with the chemical formula FeO. It consists of the chemical elementiron in the oxidation state of 2 bonded to oxygen. Its mineral form is known as wüstite. Iron(II) oxide should not be confused with rust, which usually consists of hydrated iron(III) oxide (ferric oxide). The term may be used more loosely for a non-stoichiometric compound as the ratio of the elements iron and oxygen can vary; samples are typically iron deficient with compositions ranging from Fe0.84O to Fe0.95O.
Iron(II) oxide adopts the cubic, rock salt structure, where iron atoms are octahedrally coordinated by oxygen atoms and the oxygen atoms octahedrally coordinated by iron atoms. The non-stoichiometry occurs because of the ease of oxidation of FeII to FeIII effectively replacing a small portion of FeII with two thirds their number of FeIII, which take up tetrahedral positions in the close packed oxide lattice.
Below 200 K there is a minor change to the structure which changes the symmetry to rhombohedral and samples become antiferromagnetic.
Iron(II) oxide makes up approximately 9% of the Earth's mantle. Within the mantle, it may be electrically conductive, which is a possible explanation for perturbations in Earth's rotation not accounted for by accepted models of the mantle's properties.