# Chromium(III) oxide

Chromium(III) oxide (or chromia) is an inorganic compound with the formula Cr $_2$O $_3$. It is one of the principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.

## Structure and properties

Cr $_2$O $_3$ has the corundum structure, consisting of a hexagonal close packed array of oxide anions with 2⁄3 of the octahedral holes occupied by chromium. Similar to corundum, Cr $_2$O $_3$ is a hard, brittle material (Mohs hardness 8 to 8.5). It is antiferromagnetic up to 307 K, the Néel temperature. It is not readily attacked by acids.

## Occurrence

Cr $_2$O $_3$ occurs naturally as the mineral eskolaite, which is found in chromium-rich tremolite skarns, metaquartzites, and chlorite veins. Eskolaite is also a rare component of chondrite meteorites. The mineral is named after Finnish geologist Pentti Eskola.

## Production

The Parisians Pannetier and Binet first prepared the transparent hydrated form of Cr $_2$O $_3$ in 1838 via a secret process, sold as a pigment. It is derived from the mineral chromite, (Fe,Mg)Cr $_2$O $_4$. The conversion of chromite to chromia proceeds via Na $_2$Cr $_2$O $_7$, which is reduced with sulfur at high temperatures:
    Na $_2$Cr $_2$O $_7$ + S → Na $_2$SO $_4$ + Cr $_2$O $_3$
The oxide is also formed by the decomposition of chromium salts such as chromium nitrate, or by the exothermic decomposition of ammonium dichromate.
    (NH $_4$) $_2$Cr $_2$O $_7$ → Cr $_2$O $_3$ + N $_2$ + 4 H $_2$O
The reaction has a low ignition temperature of less than 200 °C and is frequently used in “volcano” demonstrations.

## Applications

Because of its considerable stability, chromia is a commonly used pigment. It was originally called viridian. It is used in paints, inks, and glasses. It is the colorant in "chrome green" and "institutional green." Chromium(III) oxide is a precursor to the magnetic pigment chromium dioxide, by the following reaction:
    Cr $_2$O $_3$ + 3 CrO $_3$ → 5 CrO $_2$ + O $_2$
Along with many other oxides, it is used as a compound when polishing (also called stropping) the edges of knives, razors, surfaces of optical devices etc. on a piece of leather, balsa, cloth or other material. It is available in powder or wax form, and in this context it is known as "green compound".
It is used as a component of refractories due to its high melting point.
From 1962, it has been used as an inert marker in livestock intake research. However, its use in animal science research has been diminishing with the increased use of Titanium dioxide due to the latter being consider more food safe, allowing for the public sale of animals used in research trials that use an inert digestion marker.

## Reactions

Chromium(III) oxide is amphoteric. Although insoluble in water, it reacts with acid to produce salts of hydrated chromium ions such as [Cr(H $_2$O) $_6$]$^{3+}$ . It is also attacked by concentrated alkali to yield salts of [Cr(OH) $_6$]$^{3−}$ .
When heated with finely divided carbon or aluminium, it is reduced to chromium metal:
    Cr $_2$O $_3$ + 2 Al → 2 Cr + Al $_2$O $_3$
Unlike the classic thermite reaction involving iron oxides, the chromium oxide thermite creates few or no sparks, smoke or sound, but glows brightly. Because of the very high melting point of chromium, chromium thermite casting is impractical.
Heating with chlorine and carbon yields chromium(III) chloride and carbon monoxide:
    Cr $_2$O $_3$ + 3 Cl $_2$ + 3 C → 2 CrCl $_3$ + 3 CO
Chromates can be formed by the oxidation of chromium(III) oxide and another oxide in a basic environment:
    2 Cr $_2$O $_3$ + 4 MO + 3 O $_2$ → 4 MCrO $_4$