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Oxygen and chalcogen compounds

Several oxides of cobalt are known. Green cobalt(II) oxide (CoO) has rocksalt structure. It is readily oxidized with water and oxygen to brown cobalt(III) hydroxide (Co(OH)). At temperatures of 600–700 °C, CoO oxidizes to the blue cobalt(II,III) oxide (Co O), which has a spinel structure. Black cobalt(III) oxide (Co O) is also known. Cobalt oxides are antiferromagnetic at low temperature: CoO (Néel temperature 291 K) and Co O (Néel temperature: 40 K), which is analogous to magnetite (Fe O), with a mixture of +2 and +3 oxidation states.

The principal chalcogenides of cobalt are the black cobalt(II) sulfides, CoS (pyrite structure), Co S (spinel structure), and CoS (nickel arsenide structure).