Chromium(III) oxide

Chromium(III) oxide is the inorganic compound of the formula Cr2O3. It is one of principal oxides of chromium. it is a widely used precursor to other chromium compounds. In the nature it is known as rare mineral eskolaite. Very rare chromium oxyhydroxide minerals, CrO(OH) (bracewellite, guyanaite and grimaldiite) are also known.

Structure and properties
Cr2O3 adopts the corundum structure, consisting of cubic close packed oxides with 2/3 of the octahedral holes occupied by chromium. It is antiferromagnetic up to 307 K, the Neel temperature. It is an amphoteric oxide, dissolving in acids to give chromium(III) salts and in molten alkali to give chromites.

Production
The Parisians Pannetier and Binet first prepared Cr2O3 in 1838 via a secret process. It is manufactured from the mineral chromite, (Fe,Mg)Cr2O4, which is mined in southern Africa, Asia, Turkey, and Cuba. The conversion of chromite to chromia entails air oxidation to Na2Cr2O7, which is subsequently reduced with carbon or sulfur.

Chromium oxide can be converted into elemental chromium metal through the thermite reaction, although this metal oxide is much less commonly used than Fe2O3 and Fe3O4. Unlike iron oxide thermites, chromium oxide thermite creates little or no sparks, smoke or sound, but it glows with a blinding light. Because of the very high melting point of chromium, chromium thermite casting is impractical.

Pigment
It is commonly called chrome green or institutional green when used as a pigment; however it was referred to as viridian when it was first discovered.