Chromium Occurrence

Chromium Occurrence in the free state in nature was never recorded. The chief mineral from which the metal is obtained is variously known as chromite, chrome iron ore, chromoferrite, or siderochrome, Cr₂O₃.FeO, and is widely distributed, particularly in serpentine and olivine rocks. It is found in Algeria, Asia Minor, Canada, France, Greece, India, New Caledonia, Rhodesia, the Shetland Islands, Sweden, the Transvaal, Turkey, and the United States. Rhodesia is the chief producer in normal times, but in the later years of the European war the United States and India produced enormous quantities. Chromium Occurrence in iron ore, containing about 85 per cent, of Cr₂O₃.FeO, has been found in the province of Kuban, Northern Caucasus. Platiniferous chromites occur in the Ural Mountains. Small quantities of chromite are present in many meteorites. Chromite occurs in octahedra, but is generally found in a brownish-black massive form, having a granular or compact structure. It is sometimes feebly magnetic, of density 4.32 to 4.57, and is one of the heaviest as well as one of the softest minerals of the spinel group, its hardness being about 5.5 according to Molis' scale. A specimen from the Urals was found to melt at 1850° C., but the usual melting-point range is from 1545° C. to 1730° C. Magnochromite and chromopicotite are varieties of chromite. The latter occurs at Dun Mountain, New Zealand, and in the Lillooet District, British Columbia, where it is found in velvet-black, massive, coarsely granular veins, and has a density of 4.239. A chromiferous iron ore, containing up to 50 per cent, of iron and varying amounts of chromium up to about 3 per cent., is mined to a considerable extent in Greece and is also employed as a source of chromium.

Crocoite, crocoisite, chrombleispath, or rothbleierz is a natural form of lead chromate, PbCrO₄. It was discovered at Berezov in Siberia, but is also found in Brazil, Swaziland, Tasmania, and, associated with galena, in the Lydenburg district of the Transvaal. It occurs in translucent, hyacinth-red crystals of adamantine lustre; density 6.00. Crocoite crystallises in the monoclinic system, usually prismatic, its crystallographic elements being

a:b:c = 0.960342:1:0.915856; β = 77° 32' 50".

Phcenicochroite or melanochroite, of density 5.75, is a basic chromate of lead, 3PbO.2CrO₃, and is probably orthorhombic. Beresowite or here- zovite contains lead carbonate as well as the chromate. Vauquelinite, probably identical with laxmannite, is a phospho-chromate of lead and copper, 2(Pb,Cu)CrO₄.(Pb,Cu)₃(PO₄)₂, of a greenish or brownish colour, occurring in the quartz of Berezov (Urals). It forms monoclinic crystals of density 5.95:

a:b:c = 0.74977:1:1.39083; β = 69° 3'.

Chromitite is obtained as a small-grained black sand, of density 31, from Kopaonik Mountain, Serbia, and has the composition (Fe,Al)₂O₃.2Cr₂O₃. It resembles magnetite, being magnetic and crystallising in the cubic system.

Chromium mica or fuchsite possesses a brilliant green colour; chrome ochre is a bright green, clayey mineral containing Cr₂O₃. Alexandrolite, milsosin, wolchonskoite, and avalite contain hydrated oxides of chromium, aluminium, and silicon. Dietzeite contains calcium iodate and chromate.

Chromium also occurs in the following minerals: daubreelite, FeS.Cr₂S₃, massive, brittle, non-magnetic, density 5.01; redingtonite, a hydrated chromium sulphate; tarapacite, essentially a chromate of potassium; and chrome-diopside, a pyroxene from Kimberley.

A nickel-chrome spinel, NiO.Cr₂O₃, has been made artificially in small green crystals. The green colour of emerald, serpentine, possibly sapphire, and other minerals is due to the presence of compounds of chromium, which also occur in rubies, corundums, and spinels. It has been shown that the peculiar spectra of ruby and diamond are due to chromium oxide which has been compelled to vibrate in an abnormal manner, leading to the production of narrow absorption bands. Traces of chromium have been detected in the soot produced by the combustion of coal obtained from deposits at Liege. Compounds of chromium are not known to play any part in the economy of plants or animals.