Aluminium oxide

Al 2
O 3

Formula unit Lattice

Choose Lattice to see one unit cell. Then drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

5 atoms in one formula unit of an ionic lattice.

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

IUPAC name: dialuminum;tris(oxygen(2-))
Molar mass: 101.9610(50) g/mol
Atoms: 5
SMILES: [O-2].[O-2].[O-2].[Al+3].[Al+3]
InChIKey: PNEYBMLMFCGWSK-UHFFFAOYSA-N
PubChem CID: 9989226

Made of

Al O

A ruby and a sapphire are, chemically, the same compound. In nature aluminium oxide occurs as ruby, sapphire, corundum and emery, Los Alamos National Laboratory notes. Corundum is the natural oxide itself, and emery is an impure crystalline variety of it. The two gems differ in colour: ruby is an intense red and sapphire is blue, according to the US Geological Survey.

The solid pairs aluminium ions, Al³⁺, with oxygen in the form of oxide ions, O²⁻, two of the first to every three of the second. Water does not dissolve it, and mineral acids and strong alkalis attack it only with difficulty. Even so, CAMEO Chemicals, a hazard database, classes it as amphoteric: a weak acid towards bases and a weak base towards acids. HSDB, the Hazardous Substances Data Bank, records it as an electrical insulator.

Structure and bonding

The formula-unit drawing sets out five ions with nothing joining them: two aluminium ions, each with a charge of +3, and three oxide ions, each −2. Six positive charges balance six negative ones. The five make one formula unit of an ionic lattice, a tally of the crystal’s ions rather than a particle that exists on its own. Those full charges are an idealisation, which the chemical bonding article explains.

Twelve aluminium ions and 18 oxide ions belong to the unit cell drawn in the Lattice view, the block of ions repeated to build the whole crystal. That makes six formula units in the same two-to-three ratio. Every aluminium ion has 6 oxide neighbours, and every oxide ion has 4 aluminium neighbours. The counts agree with the formula: two aluminium ions with six contacts each and three oxide ions with four each both come to twelve. The Coordination button picks out one oxide ion and its four neighbours, 1.85 to 1.97 Å away.

Where it turns up

On the hardness scale that the Austrian mineralogist F. Mohs devised in 1822, sapphire stands at 9 and diamond at 10. Aluminium oxide is also made into artificial gems, and synthetic ruby and sapphire go into lasers, Los Alamos notes. The first laser used a ruby crystal, according to the US National Institute of Standards and Technology. Theodore Maiman demonstrated it in Malibu, California, at Hughes Research Laboratories in May 1960.

High-pressure sodium lamps keep their arc inside a tube of translucent alumina ceramic. The ceramic stands up to the heat and to the chemically aggressive hot sodium vapour better than lamp glass, according to RP Photonics. HSDB lists spark plugs, crucibles and laboratory ware among the oxide’s products. China led the world in making abrasive fused aluminium oxide in 2024, the US Geological Survey reports. In cosmetics it makes a product opaque or controls its viscosity.

Why it matters

About 85 per cent of the world’s bauxite goes to the Bayer process, which refines it into aluminium oxide, according to the US Geological Survey (USGS). Sodium hydroxide reacts with the ore to form sodium aluminate, from which a hydrated oxide is precipitated and then calcined, or strongly heated. The method works because the oxide’s solubility in sodium hydroxide depends on temperature, while most of the ore’s other components stay inert. Roughly 4 tonnes of dried bauxite give 2 tonnes of oxide, enough for 1 tonne of aluminium.

World output reached about 142 million tonnes in 2024 on USGS estimates, 84 million of it in China. In the United States, 69 per cent of the oxide produced went to smelters making primary aluminium. When the oxide in a smelter’s electrolysis bath runs below critical levels, the cell enters an anode effect. During one it makes no aluminium yet still consumes energy and oxide, and it releases CF₄ and C₂F₆, perfluorocarbons that the US Environmental Protection Agency calls potent greenhouse gases.

Properties

Properties of aluminium oxide Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 9989226.
Property	Value	Unit	Source
Formula (Hill order)	Al₂O₃
Molar mass	101.9610(50) g/mol	g/mol	IUPAC/CIAAW 2024
Aluminium atoms	2
Oxygen atoms	3
PubChem molecular weight	101.961 g/mol	g/mol	PubChem

Safety

As gems, ceramic parts or abrasive grit, aluminium oxide is essentially harmless. In 92.8 per cent of reports to the European Chemicals Agency it is left unclassified under the Globally Harmonized System (GHS). Its dust is the concern for people who work with the powder: it can irritate the eyes, skin and airways, and Japan’s classification adds lung damage from repeated inhalation. The Pocket Guide card of the US National Institute for Occupational Safety and Health covers workplace exposure to alpha-alumina. The PubChem Laboratory Chemical Safety Summary compiles the classifications.

Questions

Does aluminium oxide have more than one crystal form?

Yes. Aluminium oxide can take several solid forms, Chem1 notes, and CAMEO Chemicals adds that different ways of making it give different crystalline modifications. In HSDB, the main component of technical aluminium oxide is the alpha form, a white hexagonal powder, and the gamma form changes into corundum at 1200 °C. The Lattice view shows a single unit cell of a crystal: 12 aluminium ions and 18 oxide ions.

Why does aluminium oxide have such a high melting point?

Its ions carry large charges, +3 and −2, and Chem1 explains that higher ionic charges raise melting points, most of all when the ions are relatively small. HSDB gives 2030 °C for aluminium oxide and 2054 °C for its alpha form. Solids that keep their essential properties when hot are called refractories, and aluminium oxide goes into refractories, crucibles, heat-resistant fibres and furnace linings.

What is activated alumina used for?

Activated alumina is a drying agent, or desiccant, for gases and vapours in the petroleum industry, HSDB records, and it also serves in water purification. The Medical Subject Headings (MeSH) list aluminium oxide as an adsorbent, a solid that holds other substances on its surface, and as a desiccant and a catalyst. MeSH adds dental cements and refractories to the products made with it.

Is aluminium oxide dust bad for your lungs?

Japan's GHS classification lists damage to the lungs from prolonged or repeated inhalation of the dust. Severe lung scarring, or fibrosis, seen in the past in bauxite workers and aluminium smelter potroom workers is put down mainly to crystalline silica breathed alongside the dust, Haz-Map says.