Iron oxides are a group of chemical compounds composed of iron and oxygen that occur in several stable and metastable forms. They range from black magnetic minerals to red and yellow pigments and include both simple oxides and hydrated or mixed-valence phases. In geology, industry and art these compounds are important for their colors, their magnetic behavior, and as the primary ores from which elemental iron is produced.

Main forms and properties

The best known stoichiometries are iron(II) oxide (FeO, wüstite), iron(III) oxide (Fe2O3, hematite) and iron(II,III) oxide (Fe3O4, magnetite). Hematite is typically red to brown and is a common pigment; magnetite is black and strongly magnetic. Many other iron oxides and hydroxides exist in nature — for example goethite and limonite — which are often hydrated or poorly crystalline. Rust generally refers to hydrated iron(III) oxides that form when iron corrodes in the presence of water and oxygen.

Occurrence and formation

Iron oxides form by igneous, metamorphic and sedimentary processes as well as by weathering of iron-bearing minerals. Large iron ore deposits made mainly of hematite and magnetite are mined for steelmaking. Near-surface soils and sediments commonly contain ochres and brown earths rich in iron oxides produced by oxidation and hydrolysis of primary minerals.

Uses and examples

Historically and today, iron oxides are prized as stable, lightfast pigments used in paints, ceramics and cosmetics: ochre, sienna and umber are natural earth pigments whose hues come from iron oxides and hydrated oxides. Industrially, most mined iron oxide serves as feedstock for iron and steel production in blast furnaces and direct-reduction plants. Other applications include magnetic materials, polishing powders, catalysts, wastewater treatment, and some biomedical uses such as contrast agents and targeted delivery.

History, distinction and notable facts

Natural iron oxides have been used as pigments since prehistoric times; cave paintings and ancient pottery frequently employed ochres and reds derived from hematite. Magnetite’s magnetic properties were recognized early and underlie many modern technologies. Chemically, iron oxides illustrate different oxidation states of iron and can form mixed-valence compounds with distinct electrical and magnetic behavior. While many distinct iron oxide minerals and synthetic phases have been characterized, care is needed in naming hydrated or poorly crystalline materials.

Further reading and resources