cobalt (Co), chemical element, ferromagnetic metal of Group 9 (VIIIb) of the periodic table, used especially for heat-resistant and magnetic alloys.

The metal was isolated (c. 1735) by Swedish chemist Georg Brandt, though cobalt compounds had been used for centuries to impart a blue colour to glazes and ceramics. Cobalt has been detected in Egyptian statuettes and Persian necklace beads of the 3rd millennium bce, in glass found in the Pompeii ruins, and in China as early as the Tang dynasty (618–907 ce) and later in the blue porcelain of the Ming dynasty (1368–1644). The name kobold was first applied (16th century) to ores thought to contain copper but eventually found to be poisonous arsenic-bearing cobalt ores. Brandt finally determined (1742) that the blue colour of those ores was due to the presence of cobalt.

Occurrence, properties, and uses

Cobalt, though widely dispersed, makes up only 0.001 percent of Earth’s crust. It is found in small quantities in terrestrial and meteoritic native nickel-iron, in the Sun and stellar atmospheres, and in combination with other elements in natural waters, in ferromanganese crusts deep in the oceans, in soils, in plants and animals, and in minerals such as cobaltite, linnaeite, skutterudite, smaltite, heterogenite, and erythrite. In animals, cobalt is a trace element essential in the nutrition of ruminants (cattle, sheep) and in the maturation of human red blood cells in the form of vitamin B₁₂, the only vitamin known to contain such a heavy element.

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With few exceptions, cobalt ore is not usually mined for the cobalt content. Rather, it is often recovered as a by-product from the mining of ores of iron, nickel, copper, silver, manganese, zinc, and arsenic, which contain traces of cobalt. Complex processing is required to concentrate and extract cobalt from these ores. By the second decade of the 21st century, the Democratic Republic of the Congo (DRC), China, Canada, and Russia were the world’s leading producers of mined cobalt. The largest producer of refined cobalt, however, was China, which imported vast additional amounts of cobalt mineral resources from the DRC. (For additional information on the mining, refining, and recovery of cobalt, see cobalt processing.)

Polished cobalt is silver-white with a faint bluish tinge. Two allotropes are known: the hexagonal close-packed structure, stable below 417 °C (783 °F), and the face-centred cubic, stable at high temperatures. It is ferromagnetic up to 1,121 °C (2,050 °F, the highest known Curie point of any metal or alloy) and may find application where magnetic properties are needed at elevated temperatures.

Cobalt is one of the three metals that are ferromagnetic at room temperature. It dissolves slowly in dilute mineral acids, does not combine directly with either hydrogen or nitrogen, but will combine, on heating, with carbon, phosphorus, or sulfur. Cobalt is also attacked by oxygen and by water vapour at elevated temperatures, with the result that cobaltous oxide, CoO (with the metal in the +2 state), is produced.

Natural cobalt is all stable isotope cobalt-59, from which the longest-lived artificial radioactive isotope cobalt-60 (5.3-year half-life) is produced by neutron irradiation in a nuclear reactor. Gamma radiation from cobalt-60 has been used in place of X-rays or alpha rays from radium in the inspection of industrial materials to reveal internal structure, flaws, or foreign objects. It has also been used in cancer therapy, in sterilization studies, and in biology and industry as a radioactive tracer.

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Most of the cobalt produced is used for special alloys. A relatively large percentage of the world’s production goes into magnetic alloys such as the Alnicos for permanent magnets. Sizable quantities are utilized for alloys that retain their properties at high temperatures and superalloys that are used near their melting points (where steels would become too soft). Cobalt is also employed for hard-facing alloys, tool steels, low-expansion alloys (for glass-to-metal seals), and constant-modulus (elastic) alloys (for precision hairsprings). Cobalt is the most satisfactory matrix for cemented carbides.

Finely divided cobalt ignites spontaneously. Larger pieces are relatively inert in air, but above 300 °C (570 °F) extensive oxidation occurs.

Compounds

In its compounds cobalt nearly always exhibits a +2 or +3 oxidation state, although states of +4, +1, 0, and −1 are known. The compounds in which cobalt exhibits the +2 oxidation state (Co²⁺, the ion being stable in water) are called cobaltous, while those in which cobalt exhibits the +3 oxidation state (Co³⁺) are called cobaltic.

Both Co²⁺ and Co³⁺ form numerous coordination compounds, or complexes. Co³⁺ forms more known complex ions than any other metal except platinum. The coordination number of the complexes is generally six.

Cobalt forms two well-defined binary compounds with oxygen: cobaltous oxide, CoO, and tricobalt textroxide, or cobalto-cobaltic oxide, Co₃O₄. The latter contains cobalt in both +2 and +3 oxidation states and constitutes up to 40 percent of the commercial cobalt oxide used in the manufacture of ceramics, glass, and enamel and in the preparation of catalysts and cobalt metal powder.

One of the more important salts of cobalt is the sulfate CoSO₄, which is employed in electroplating, in preparing drying agents, and for pasture top-dressing in agriculture. Other cobaltous salts have significant applications in the production of catalysts, driers, cobalt metal powders, and other salts. Cobaltous chloride (CoCl₂∙6H₂O in commercial form), a pink solid that changes to blue as it dehydrates, is utilized in catalyst preparation and as an indicator of humidity. Cobaltous phosphate, Co₃(PO₄)₂∙8H₂O, is used in painting porcelain and colouring glass.