Iron(II) oxide

Iron(II) oxide, also known as ferrous oxide or ferrous iron, is one of the iron oxides. It is a black-colored powder with the chemical formula FeO. It consists of the chemical element iron in the oxidation state of 2 bonded to oxygen. Its mineral form is known as wüstite. The latter term is often used synonymously to ferrous oxide. Iron(II) oxide is an example of a non-stoichiometric compound and the ratio of the elements iron and oxygen can vary due to crystallographic vacancies. Iron (II) oxide is also very unstable and it easily susceptible to corroding into Iron (III) Oxide. Iron(II) oxide should not be confused with rust, which usually consists of hydrated iron(III) oxide (ferric oxide).

Iron(II) oxide is used as a pigment. It is FDA-approved for use in cosmetics and it is used in some tattoo inks.