 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 ) 3 ) . At temperatures of 600 – 700 ° C , CoO oxidizes to the blue cobalt ( II , III ) oxide ( Co3O4 ) , which has a spinel structure . Black cobalt ( III ) oxide ( Co2O3 ) is also known . Cobalt oxides are antiferromagnetic at low temperature : CoO ( Néel temperature 291 K ) and Co3O4 ( Néel temperature : 40 K ) , which is analogous to magnetite ( Fe3O4 ) , with a mixture of + 2 and + 3 oxidation states . 
