Iron(III) oxide

Fe 2
O 3

5 atoms in one formula unit of an ionic lattice.

The drawing shows one formula unit of an ionic lattice, not a molecule; iron(III) oxide has no molecular shape.

IUPAC name: bis(iron(3+));tris(oxygen(2-))
Molar mass: 159.6870(70) g/mol
Atoms: 5
SMILES: [O-2].[O-2].[O-2].[Fe+3].[Fe+3]
InChIKey: LIKBJVNGSGBSGK-UHFFFAOYSA-N
PubChem CID: 14833

Made of

Steel-grey crystals known as specular iron or looking-glass ore, and the red powders called red ochre and rouge, are all varieties of iron(III) oxide. HSDB, the Hazardous Substances Data Bank quoted on PubChem, describes the natural mineral in shades from brilliant black through blackish red to brick red. Its lustre runs from metallic to dull. That mineral is hematite.

Each formula unit joins two iron ions to three ions of oxygen, giving a molar mass of 159.69 g/mol. HSDB ranks hematite as the most important ore of iron. Red iron ore is mostly this oxide and holds about 70 per cent iron. Water does not dissolve it. Hydrochloric and sulfuric acids do, while nitric acid takes up only a little. Like the oxides of transition metals generally in Chem1’s account, it shows no acidic properties in water.

Structure and bonding

Five ions and no bonds make up the drawing: two iron ions marked +3 and three oxide ions marked −2, whose charges cancel. The group is one formula unit of a lattice, the ratio in which the ions occur; the solid holds no separate five-ion units.

HSDB describes the real solid as hexagonal crystals, red-brown to steel grey. Its alpha form occurs in nature as hematite and its gamma form as maghemite. Chem1 counts the oxygen compounds of transition metals among solids with complex, extended structures. It places most oxides of metals beyond Group 2 among solids whose structures are dominated by attractions other than those between ions. The +3 and −2 are oxidation numbers: the charges if oxygen, which pulls harder on shared electrons, took them all.

Both everyday names encode that +3, for iron is multivalent, Chem1’s word for a metal with ions of more than one charge. The older system marks the lower charge with the ending -ous and the higher with -ic, which makes this ferric oxide. The Stock system writes the charge, strictly the oxidation number, as a Roman numeral: iron(III) oxide. Its IUPAC name, bis(iron(3+));tris(oxygen(2-)), names both ions with their charges.

Where it turns up

Hematite often shows up as black sand along beaches and stream banks, Los Alamos notes. It is a common mineral of intensely weathered soils in warm, humid climates, where it can form directly as iron-bearing minerals oxidise. A US Geological Survey report notes that the redder a soil, the more hematite it tends to hold.

Minnesota’s Mesabi Range was mined for hematite, a rusty-red ore with the look of gravel. From 1890 to about 1980 its mines removed 2.5 billion tons, and by the 1980s those natural ores were largely gone. The mineral lies on Mars as well. NASA’s Opportunity rover landed on Meridiani Planum in January 2004, where parts of the surface hold up to 20 per cent grey crystalline hematite. It sits in tiny spheres that scientists nicknamed blueberries. On NASA’s account, strongly acidic groundwater soaked a sandstone rich in goethite, another iron-bearing mineral, and turned that goethite into hematite.

Why it matters

Iron oxide pigments colour concrete block, brick, ready-mixed concrete and roofing tiles. They are colour-stable, cheap and nontoxic, which the US Geological Survey gives as the reason they can tint large, low-value products economically in black, brown, red and yellow. Many organic colourants, by contrast, fade in sunlight. Painters of rock art used hematite for red, calcium carbonate for white and charcoal for black, the US National Park Service records.

Jeweller’s rouge is this oxide as a fine red powder, and it polishes glass, precious metals and diamonds. NIOSH, the US National Institute for Occupational Safety and Health, describes rouge as usually used in cakes or soaked into paper or cloth. Hematite is one of the alternatives to barite in drilling fluids. As an iron supplement in animal feed it is worthless: trials in rats, chicks, cattle and sheep confirmed that it is no use as a source of iron, HSDB reports.

Properties

Properties of iron(III) oxide Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 14833.
Property	Value	Unit	Source
Formula (Hill order)	Fe₂O₃
Molar mass	159.6870(70) g/mol	g/mol	IUPAC/CIAAW 2024
Iron atoms	2
Oxygen atoms	3
PubChem molecular weight	159.69 g/mol	g/mol	PubChem

Safety

Iron(III) oxide meets no Globally Harmonized System (GHS) hazard criteria in 88 per cent of reports to the European Chemicals Agency. The rest class it as a skin irritant that can seriously damage the eyes. Rouge, its fine powder, irritates the eyes, skin and airways, and dust and fume from mining, smelting, polishing and welding can cause siderosis, a benign dust disease of the lungs. NIOSH’s cards for rouge and iron oxide dust give the workplace limits, and the PubChem Laboratory Chemical Safety Summary gathers the classifications.

Questions

What does the drawing of iron(III) oxide show?

It shows one formula unit of Fe₂O₃: two Fe³⁺ ions and three O²⁻ ions, five charged particles with no bonds drawn and charges that add up to zero. The real substance is a lattice with no five-ion particles in it. The Hazardous Substances Data Bank describes its crystals as hexagonal, hard at about 6 to 6.5 on the Mohs scale of scratch hardness, and dense at 5.25 g/cm³.

What makes Mars red?

NASA's Mars Facts page attributes the reddish look to oxidation, or rusting, of iron in the planet's rocks, regolith (the loose Martian soil) and dust. A 2025 study reported by NASA points to one iron oxide, ferrihydrite, as the source of the colour. Ferrihydrite forms in cool water and at lower temperatures than minerals considered before it, such as hematite. Iron(III) oxide itself is present too, as grey crystalline hematite.

Why is hematite red in some places and black or grey in others?

The Hazardous Substances Data Bank ties the colour and look of iron(III) oxide to three things: the size of its particles, their shape, and the amount of water combined with them. In synthetic pigment the shade is set at manufacture, by the iron compound chosen and the reaction conditions. Synthetic grades also outdo natural pigment in uniformity, strength and brilliance.

Is iron oxide used in makeup and food?

Yes, as a colour. The US Food and Drug Administration has listed synthetic iron oxide as a colour additive for drugs from 1966, iron oxides for cosmetics generally, eye-area use included, from 1977, and for contact lenses from 1986. The agency also approves synthetic iron oxide, E172, as a colour for human food, under uses and restrictions set in federal regulations.