Chromium(III) oxide (or chromia) is an

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistr ...

with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

. It is one of the principal oxides of

chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...

and is used as a pigment. In nature, it occurs as the rare mineral

eskolaite Eskolaite is a rare chromium oxide mineral (chromium(III) oxide Cr2O3). Discovery and occurrence It was first described in 1958 for an occurrence in the Outokumpu ore deposit of eastern Finland. It occurs in chromium bearing tremolite skarns, met ...

Structure and properties

has the

corundum Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the pre ...

structure, consisting of a

hexagonal close packed In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or lattice). Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occ ...

array of oxide anions with of the octahedral holes occupied by chromium. Similar to corundum, is a hard, brittle material (

Mohs hardness The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by t ...

8 to 8.5). It is

antiferromagnetic In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins (on different sublattices) pointing in opposite directions. ...

up to 307 K, the

Néel temperature In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...

. It is not readily attacked by acids.

Occurrence

occurs naturally as the mineral

, which is found in chromium-rich

tremolite Tremolite is a member of the amphibole group of silicate minerals with composition: Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and fer ...

skarns, metaquartzites, and

chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...

veins. Eskolaite is also a rare component of chondrite meteorites. The mineral is named after Finnish geologist

Pentti Eskola Pentti Elias Eskola (8 January 1883 – 6 December 1964) was a Finnish geologist who specialised in the petrology of granites and developed the concept of metamorphic facies. He won the Wollaston Medal in 1958, the Vetlesen Prize in 1964, and ...

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 Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can su ...

, . The conversion of chromite to chromia proceeds via , which is reduced with sulfur at high temperatures:Gerd Anger, Jost Halstenberg, Klaus Hochgeschwender, Christoph Scherhag, Ulrich Korallus, Herbert Knopf, Peter Schmidt, Manfred Ohlinger, "Chromium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. : + S → + The oxide is also formed by the decomposition of chromium salts such as chromium nitrate, or by the exothermic decomposition of

ammonium dichromate Ammonium dichromate is an inorganic compound with the formula (NH4)2Cr2O7. In this compound, as in all chromates and dichromates, chromium is in a +6 oxidation state, commonly known as hexavalent chromium. It is a salt consisting of ammonium ions a ...

. : → + + 4 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 Viridian is a blue-green pigment, a hydrated chromium(III) oxide, of medium saturation and relatively dark in value. It is composed of a majority of green, followed by blue. Specifically, it is a shade of spring green, which places the color b ...

. 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 Chromium dioxide or chromium(IV) oxide is an inorganic compound with the formula CrO2. It is a black synthetic magnetic solid. It once was widely used in magnetic tape emulsion. With the increasing popularity of CDs and DVDs, the use of chrom ...

, by the following reaction: : + 3 → 5 + 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 In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...

due to its high melting point.

Reactions

Chromium(III) oxide is

amphoteric In chemistry, an amphoteric compound () is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used. One type of amphoteric species are amphiprot ...

. Although insoluble in water, it reacts with acid to produce salts of hydrated chromium ions such as .R. Scholder "Sodium Hexahydroxochromate(III)" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 2, 1688ff. 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: : + 2 Al → 2 Cr + 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 Chromium(III) chloride (also called chromic chloride) describes any of several chemical compounds with the formula CrCl3, where can be 0, 5, and 6. The anhydrous compound with the formula CrCl3 is a violet solid. The most common form of the tric ...

and

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...

: : + 3 + 3 C → 2 + 3 CO Chromates can be formed by the oxidation of chromium(III) oxide and another oxide in a basic environment: : 2 + 4 MO + 3 → 4

References