Chromium(III) oxide (or chromia) is the inorganic compound of the formula Cr
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.
3D model (JSmol)
|Molar mass||151.9904 g/mol|
|Appearance||light to dark green, fine crystals|
|Melting point||2,435 °C (4,415 °F; 2,708 K)|
|Boiling point||4,000 °C (7,230 °F; 4,270 K)|
|Solubility in alcohol||insoluble in alcohol, acetone, acids|
Refractive index (nD)
Std enthalpy of
|US health exposure limits (NIOSH):|
|TWA 1 mg/m3|
|TWA 0.5 mg/m3|
IDLH (Immediate danger)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
3 adopts the corundum structure, consisting of a hexagonal close packed array of oxide anions with ⅔ of the octahedral holes occupied by chromium. Similar to corundum, Cr
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.
3 occurs naturally in 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.
The Parisians Pannetier and Binet first prepared the transparent hydrated form of Cr
3 in 1838 via a secret process, sold as a pigment. It is derived from the mineral chromite, (Fe,Mg)Cr
4. The conversion of chromite to chromia proceeds via Na
7, which is reduced with sulfur at high temperatures:
The oxide is also formed by the decomposition of chromium salts such as chromium nitrate or by the exothermic decomposition of ammonium dichromate.
The reaction has a low ignition temperature of less than 200 °C and is frequently used in “volcano” demonstrations.
Because of its considerable stability, chromia is a commonly used pigment and was originally called viridian. It is used in paints, inks, and glasses. It is the colourant in "chrome green" and "institutional green." Chromium(III) oxide is a precursor to the magnetic pigment chromium dioxide, according to the following reaction:
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 alternatively known as "green compound".
Chromium(III) oxide is amphoteric. Although insoluble in water, it dissolves in acid to produce hydrated chromium ions, [Cr(H
which react with base to give salts of [Cr(OH)
. It dissolves in concentrated alkali to yield chromite ions.
When heated with finely divided carbon it can be reduced to chromium metal with release of carbon dioxide. When heated with finely divided aluminium it is reduced to chromium metal and aluminum oxide:
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.
Chromates can be formed by the oxidation of chromium(III) oxide and another oxide in a basic environment: