Iron(III) oxide

IUPAC : Iron(III) oxide

OxideMineralIndustrialMineral/geological

The most stable iron(III) oxide and the main iron ore mined worldwide (hematite). Responsible for the characteristic red colour of the Martian surface, red ochres and rust.

Scroll to activate 3D

Physical properties

Molar mass159.688 g/mol

State at 20 °Csolid

Density5.24 g/cm³

Melting point1838.00 K (1565 °C)

Boiling point—

Solubility (H₂O)insoluble

Structure

Crystal systemRhombohedral

3D render modeBall-and-stick

Detailed description

Iron(III) oxide Fe₂O₃, in its crystalline hematite form (from the Greek haima, blood), is probably the colour most present in the human geological imagination. It tints the Martian surface red (the Romans already called Mars the "red planet" without knowing its composition), the prehistoric ochres of Lascaux and Altamira cave paintings, the rust eating unprotected steel, and the sedimentary cliffs of Colorado, Petra and the English Devon coast. On Earth, it is the dominant iron ore — about 1.8 Gt mined annually, of which Australia and Brazil supply over 60 %.

Crystallographically, Fe₂O₃ has several polymorphs. The most stable, α-Fe₂O₃ (hematite), has a corundum-type rhombohedral structure where each iron atom is octahedrally coordinated by six oxygens. The metastable γ-Fe₂O₃ (maghemite) is ferrimagnetic — it built the fortune of magnetic recording tape in the 1960s-1990s. Above ~800 °C, maghemite irreversibly converts to hematite.

Industrially, conversion to metallic iron happens in the blast furnace: Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂ at ~1500 °C with coke as the reducer. This route produces about 1.9 Gt of steel per year but emits ~2 Gt of CO₂ — roughly 7 % of global emissions. Emerging "hydrogen steelmaking" aims to replace coke with green H₂: Fe₂O₃ + 3 H₂ → 2 Fe + 3 H₂O, removing direct CO₂ emissions and providing the main hope for deep decarbonisation of the sector. Sweden's HYBRIT startup delivered its first tonnes in 2021.