Element Molybdenum, Mo, Transition Metal
|In view of their similarity in appearance, it is not surprising that in early times such substances as molybdenite and plumbago should have been confused with galena. That such was the case is reflected in the names " black lead " and "molybdoena", and the difference between the two substances was first pointed out in 1778-79 by Scheele in his treatises on " Molybdoena " and " Plumbago." During his investigations on the composition of the former, Scheele heated it with nitric acid and obtained an acid-forming oxide; the subject was studied by Bergman (1781) and Hjelm, who isolated the metal in 1782. The pure metal, free from carbon, was first obtained by Berzelius by reduction of the oxide with hydrogen. The true composition of wulfenite was first demonstrated by Klaproth in 1797. |
The most important researches on molybdenum compounds previous to 1880 were those of Svanberg and Struve (1848), and Ullik (1867), on the molybdates, and of Blomstrand (1857) on the halogen compounds. During the last half century much attention has been paid to the chemistry of molybdenum, and although our knowledge of the lower oxidation products of the element is still restricted, the nature of the many series of complex compounds has to a great extent been elucidated.
The word "molybdenum" comes from the Greek molybdos, meaning lead and the mineral lead glance. This was confused with molybdenite and graphite until late Medieval. All three minerals were previously known as molybdaena. In 1758, Cronstedt presumed that molybdenite and graphite are different minerals; 20 years late Scheele proved it obtaining MoO3, which he called Wasserbleyerde or molybdenum acid (Acidum molybdaenae). In 1790 Peter Jacob Hjelm reported about successful impure Molybdenum isolation using reducing the oxide by carbon. Pure molybdenum was separated by Berzelius in 1817.
|Molybdenum is a typical rare element with crustal abundance 1.1×10-4% by mass. Total amount of Molybdenum minerals is 15; most of them (various molybdates) are formed in biosphere. In magmatic processes Molybdenum is associated with acid or granitic magma. Molybdenum concentration in mantle is small, in ultrabasic rocks it is 2×10-5 %. Molybdenum accumulation is connected with abyssal hot waters, from which it is settled as MoS2, the main commercial mineral, forming hydrothermal deposits; H2S is the main precipitation agent.|
Molybdenum geochemistry in biosphere is closely related with living matter and products of its decay. Average Molybdenum abundance in organisms is 1×10-5 %. On the surface of the Earth, especially in alkaline environment, Molybdenum (IV) easily oxidizes yielding molybdates. Molybdenum easily migrates in dry climate landscape, accumulating in salt lakes by evaporating (until 1×10-3 %) as well as in salt marshes. In moist climate and acid soils Molybdenum is non-mobile; in such environment molybdenum fertilizers are required, as for leguminous crops.
The average content of Molybdenum in river water is 10-7-10-8 %. Being transported with the river outflow into the ocean, Molybdenum is partially accumulated in sea water, at an average abundance of 1×10-6 %, and partially precipitates concentrating in clayey mud, enriched by organic matter and H2S.
Except molybdenum ores other Molybdenum-containing rocks and ores, such as copper and copper-lead-zinc ores also may appear as Molybdenum source.
Molybdenum plays a complex biological role. It is considered as a necessary microelement. It is found in plants green material. Leguminous crops are enriched by Molybdenum. This element is a constituent part of enzyme xanthine oxidase. It is an essential part of metabolism: the lack of xanthine oxidase leads to high concentration of xanthine in blood and can cause health problems such as renal failure.
Of the minerals containing molybdenum, the most important are molybdenite and wulfenite. The former consists essentially of the sulphide MoS2, and occurs in a crystalline (hexagonal) form in granite, gneiss, granular limestone, etc. Its density is 4.75. It was formerly confused with graphite, which it resembles in appearance, but the difference was pointed out by Scheele in 1778-79. An artificial "molybdenite" has been prepared. Molybdenite is widely, though sparingly, distributed, and is found in the following countries in the British Empire: Australia (chiefly Queensland and New South Wales), New Zealand, Canada and Newfoundland, Virgin Isles, Ceylon, Federated Malay States (not worked), India (not worked), Union of S. Africa (Natal, Transvaal), United Kingdom (Cornish mines, Inverness, Leicester, Cumberland). It occurs also in Austria, France, Germany, Russia, Norway, Sweden, the United States, Japan, and Mexico. Wulfenite, melinose, or yellow lead spar, is lead molybdate, PbMoO4. It occurs in veins with other lead ores in Austria, Hungary, Scotland, Siberia, and the United States. The brilliant red crystals of adamantine lustre, which have a density of 6.85, belong to the tetragonal system. Wulfenite is a member of the scheelite group of minerals.
Molybdite or molybdenum ochre forms orthorhombic crystals, and occurs with molybdenite, from which it is probably derived. It consists essentially of the trioxide MoO3, but analysis has shown that its composition is probably expressed by the formula Fe2O3.3MoO3.7½H2O; it being, in fact, a hydrated ferric molybdate. The sample examined was of a yellow colour, possessed a fibrous structure and a silky lustre, and was pleochroic.
Other minerals containing molybdenum are very rare. Powellite, calcium molybdate, CaMoO4, containing calcium tungstate, CaWO4, is a brittle mineral from Idaho. The crystals belong to the tetragonal system; density 4.53. Belonesite, which occurs in Vesuvian lava,5 is essentially magnesium molybdate, MgMoO4; pateraite, a molybdate of iron and cobalt; eosite, a vanado-molybdate of lead; while one of the constituents of achrematite is lead molybdate. Ilsemannite, a blue oxide, and molybdates of iron and uranium, are also found in nature. The element has been detected in soot and dust produced by the combustion of coal obtained from the Liege district.