The primary cobalt minerals, chiefly arsenides, sulpharsenides, and sulphides, break down in the zone of oxidation. The sulphate is soluble, but the pink arsenate, erythrite (Co3As2O8H2O) is stable and constitutes "cobalt bloom," a conspicuous guide to cobalt. A good share of the world's supply of the metal comes from oxidized ores in the Belgian Congo, in which the cobalt is said to occur as heterogenite, a black hydrous oxide, and in asbolite, a cobalt-manganese oxide, both of in-definite composition.

The oxidation and migration of cobalt needs further investigation. Judging from occurrences in the state of Goiaz (Brazil)," it would appear that cobalt is less mobile than nickel, at least in the presence of manganese, since oxidized cobalt ore forms cappings a fraction of a meter thick with veinlets extending down one, two, or at most ten meters, over supergene nickel deposits. Likewise, in New Caledonia, asbolite ores associated with serpentine rest on the bedrock and are capped by iron oxides, whereas the nickel occurs in stringers in the upper part of the serpentine rock-mass.