Chromium(III) oxide

Chromium(III) oxide (or chromia) is an inorganic compound with the formula Cr2O3. It is one of the principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite. Chromium(III) oxide (or chromia) is an inorganic compound with the formula Cr2O3. It is one of the principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite. Cr2O3 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, Cr2O3 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. Cr2O3 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. The Parisians Pannetier and Binet first prepared the transparent hydrated form of Cr2O3 in 1838 via a secret process, sold as a pigment. It is derived from the mineral chromite, (Fe,Mg)Cr2O4. The conversion of chromite to chromia proceeds via Na2Cr2O7, 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. 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: 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'. Chromium(III) oxide is amphoteric. Although insoluble in water, it reacts with acid to produce salts of hydrated chromium ions such as 3+. It is also attacked by concentrated alkali to yield salts of 3−.