Glossary

Iron oxide is a compound formed when iron (Fe) reacts with oxygen (O₂), creating a family of chemical compounds that consist of iron and oxygen in various ratios. These compounds occur naturally as minerals and artificially through corrosion, oxidation, or controlled synthesis, and they are among the most common inorganic materials on Earth.

Chemically, iron oxides are represented by several forms, depending on the oxidation state of iron and the environment in which they form. The three most significant types are:

1. Iron(II) oxide (FeO) — also known as wüstite, this is a black or dark gray compound where iron is in the +2 oxidation state. It typically forms under low-oxygen conditions or as an intermediate product in high-temperature reactions such as steelmaking. FeO is unstable in air and tends to oxidize further.

2. Iron(III) oxide (Fe₂O₃) — commonly called hematite or rust, this is a reddish-brown compound in which iron is in the +3 oxidation state. It is the most stable and widespread form of iron oxide, found both in nature (as hematite ore) and as the end product of corrosion when iron or steel reacts with oxygen and moisture. The reaction can be summarized as:
4Fe + 3O₂ → 2Fe₂O₃

3. Iron(II,III) oxide (Fe₃O₄) — also known as magnetite, this compound contains both Fe²⁺ and Fe³⁺ ions and has a black, magnetic appearance. It forms under moderate oxygen conditions and is used in magnets, pigments, and recording materials. The oxidation process can be written as:
3Fe + 2O₂ → Fe₃O₄

These oxides are not only important in geology and metallurgy but also have widespread industrial and technological uses. For instance, iron oxides serve as pigments (red, yellow, and black iron oxides) in paints, ceramics, and cosmetics; as polishing agents (jeweler’s rouge); and as magnetic materials in electronics and data storage.

In corrosion, iron oxides are often undesirable, forming the flaky, porous “rust” that compromises structural integrity. However, controlled oxidation is sometimes beneficial—such as in passivation layers on stainless steel, where a stable oxide film protects against further corrosion.