Chromium(III) oxide
Chromium(III) oxide (or chromia) is an inorganic compound with the formula . 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. has the corundum structure, consisting of a hexagonal close packed array of oxide anions with of the octahedral holes occupied by chromium. Similar to corundum, 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. 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 in 1838 via a secret process, sold as a pigment. It is derived from the mineral chromite, . The conversion of chromite to chromia proceeds via , 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. 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: 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 it's high melting point. Reactions. Chromium(III) oxide is amphoteric. Although insoluble in water, it reacts with acid to produce salts of hydrated chromium ions such as . It is also attacked by concentrated alkali to yield salts of . When heated with finely divided carbon or aluminium, it is reduced to chromium metal: 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: Chromates can be formed by the oxidation of chromium(III) oxide and another oxide in a basic environment: