Element Cobalt, Co, Transition Metal

Cobalt compounds have been known for millennia. Cobalt glass ingots have been recovered from Egyptian ancientries such as jar dating to the 15^th century B.C., in coloring of which cobalt salts had been used, as well as in the cobalt-contained glassbricks. Ancient Assyrians and Babylonians used to produce blue paints which were used for ceramics glazing. Perhaps bismuth- and cobalt-contained sapphire was the initial material from which the names of blue paints came.

Swedish chemist, director of the chemical laboratory of the Council of Mines, Stockholm George Brandt (1694-1768) is credited with isolating and describing cobalt in 1735. As he wrote in his dissertation entitled "About half-metals": "As there are six kinds of metals, so I have also shown with reliable experiments... that there are also six kinds of half-metals: a new half-metal, namely Cobalt regulus". He showed that cobalt was the source of the blue color in glass, which previously had been attributed to the bismuth found with cobalt.

According to some sources, pure cobalt was separated by Berzelius.

The cobalt abundance in lithosphere is 1.8x10^-3 mass %. It migrates in magmas of the Earth's crust and in hot and cold waters. Due to the process of magmatic differentiation cobalt is deposited mostly in upper mantle. Magmatic processes are also responsible for so-called segregation deposits of cobalt ores. In hydrothermal deposits cobalt is associated with Nickel, Arsenic, Sulphur and Copper. In total approximately 30 cobalt ores are known.

Cobalt disperses in biosphere; however some plants are able to concentrate it participating in origin of cobalt deposits. In upper layers of Earth crust a distinctive differentiation of cobalt concentration may be observed 2x10^-3% in slates, 3x10^-5% in sandstones, 1x10^-5 in limestone. Sand soils of forest areas contain lower amount of cobalt. Surface waters are also poor by cobalt the ocean concentration does not exceed 5x10^-8%. Being very poor water migrant cobalt is easily transferred to sedimentary rocks in which it is absorbed by manganese hydroxides, clays, and other highly dispersed adsorbents.

Cobalt is a microelement, which means that it permanently occurs in biological tissues. Some terrestrial plants and seaweeds are able to concentrate cobalt. Being a constituent part of vitamin B₁₂, also called cobalamin, cobalt participates in the most important functions such as haematogenesis, in processes in neurosystem, in liver, etc. Cobalt is important for enzymatic processes of fixation of air nitrogen by legume bacteria. An average human body (70 kg) contains approximately 14 mg of cobalt.

A fairly common mineral is the ortho-arsenate, Co₃(AsO₄)₂.8H₂O, known as erythrite, or cobalt bloom. It is crimson in colour and isomorphous with vivianite (ferrous phosphate). Other minerals are asbolan, earthy cobalt, or wad, (CoMn)O.2MnO₂.4H₂O; and linnceite, (CoNiFe)₃S₄. The former contains anything from 2 to 20 per cent, of cobalt, and is found in New Caledonia and in Spain. Bismutosmaltite, Co(AsBi)₃. Carrollite, Co₂CuS₄. Willyamite, NiCoSb₂S₂. Jaipurite, CoS.

Mineral	Chemical composition	Colour	Density	Hardness (Mohs' scale)	Crystallographic system
Absolan or asbolite	See Wad
Bieberite or cobalt vitriol	CoSO4.7H2O	Flesh - colour	1.9	-	-
Bismuto-cobaltite	Co(AsBi)3	-	-	-	-
Carrollite	Co2CuS4	Steel-grey	4.85	5.5	Cubic
Cobalt bloom	See Erythrite
Cobalt glance	See Cobaltite
Cobaltite	CoAsS	Silver-white	6.2	5.5	Cubic
Earthy cobalt	See Wad
Erythrite or erythine	Co3(AsO4)2.8H2O	Crimson	2.95	1.5-2.5	Monoclinic
Glaucodote	(FeCo)AsS	Tin-white	5.95	5	Rhombic
Jaipurite or Syepoorite	CoS	Grey	5.45	-	-
Linnseite	(CoNiFe)3S4	Tin-white	5.5	4.8-5	Cubic
Saffiorite	CoAs2	Tin-white	6.9-7.3	4.5-5	Rhombic
Skutterudite	CoAs3	Tin-white	6.7	6	Cubic
Smaltite	CoAs2	Tin-white	6.2	5.5	Cubic
Wad	(CoMn)O.2MnO2.4H2O	Black or blue-black	0.5-6	3-4.3	Amorphous

Cobalt District, Ontario, Canada, contains the most important ores of cobalt in the form of arsenides, associated with nickel and silver, the last-named metal rendering the ores very valuable. Their cobalt content ranges from 9 to 18 per cent.

The presence of cobalt in the sun's photosphere has been determined spectroscopieally.

Metallic Cobalt can be easily obtained as a powder by heating the oxide in a current of hydrogen. In fused masses it is most easily obtained by reduction with aluminium, according to the method of Goldschmidt. It is a tenacious metal, which can be readily polished, and which exhibits a high lustre. In the metallic state it has as yet found no application in the arts.

In most acids it dissolves only very slowly, with evolution of hydrogen, but dissolves readily in nitric acid. The solutions which are formed are coloured red, irrespective of the nature of the acid; it is to be concluded from, this that the red colour is due to cobaltion. The cobaltion, Co^••, contained in the salts is divalent, and in its general behaviour is similar to diferrion.

With alkalis, its salts yield a blue-violet precipitate of cobalt hydroxide, Co(OH)₂, which is converted into its anhydride, green cobaltous oxide, CoO, on being heated out of contact with oxygen. At a red heat it takes up oxygen from the air, and an oxide, Co₃O₄, corresponding to magnetic iron ore, is formed, which is again converted into the monoxide at a white heat.

Cobalt hydroxide does not dissolve in excess of alkalis (except in traces, when the solutions are very concentrated), but readily does so in solutions of ammonium salts. The reaction is, in the first instance, similar to that in the case of magnesium hydroxide; but if a large excess of ammonia is added, the red colour changes to a yellow-brown, which shows that a new, complex compound has been formed. If the liquid is diluted with much water, blue cobalt hydroxide separates out as a flocculent precipitate. As in the case of manganese, the ammoniacal solution absorbs oxygen from the air, whereby complex salts are formed which will be mentioned later.

Of the salts of cobalt, cobalt nitrate, Co(NO₃)₂, is the best known; it is a readily soluble salt crystallising with 6H₂O, and is used in analytical chemistry.