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Element Thorium, Th, Actinide or Actinoid

Thorium History

In 1815, Jons Jakob Berzelius, examining samples ores from mines of Fahlun (Falun) mines, Sweden, obtained a material which he hastily called thorium. In spite of the fact that the conclusion was wrong, it was considered indisputable because of the great scientist's authority. The error was found by Berzelius himself after 10 years. In 1828 Prof. Esmark, having found a mineral in syenites from the island of Lovo, Norway, was unable to identify it as any known mineral, and he sent a specimen to Berzelius for examination. A chemical analysis of this mineral by Berzelius demonstrated that it contained of a silicon earth as well as of oxide of a new element which was again named after Thor, the ancient Scandinavian god of thunder. Berzelius failed in attempt of isolating the metal thorium. It was done by Lars Frederik Nilson in 1882. Thorium is a member of actinide (or actinoid) group of chemical elements.

Thorium Occurrence

Thorium-232 isotope is the most long-lived and is found in nature. Its crustal abundance is 8×10-4% by mass. Around 120 thorium-containing minerals are known, the most important of which are thorite ThSiO4, thorianite (Th,U)O2 and monazite which is the main industrial source of thorium. It is contained also in ilmenite, edisonite, cassiterite and rare earth elements ores. Thorium-230, a product of uranium decay is also found in nature. The commercial deposits were estimated in the beginning of 1980 1 million metric tons. The main deposits are located in India, Canada, USA, Norway and Brazil.

Thorium is constantly present in plant and animal tissues. Thorium accumulation coefficient which equals the ratio of concentration in organisms and environmental concentration is 1250 in sea plankton, 10 in benthic seaweed, - 50-300 in Invertebrata soft tissues and 100 in fish. The concentration in fresh water mollusks (Unio mancus) varies from 3×10-7 to 1×10-5%, and in sea animals from 3×10-7 to 3×10-6%.

Thorium is mostly absorbed by liver and spleen as well as by bone marrow, lymphatic glands and suprarenal capsules. It is poorly soaked by gastrointestinal tract.

Thorium was first discovered in minerals occurring in Scandinavia. The chief of these are thorite and its gem-variety orangite, which are silicates of very complex composition. Thorite contains nearly 60 per cent, of thoria, about 20 per cent, of silica, and smaller quantities of uranic, ferric, manganic, copper, magnesium, potassium, sodium, lead, tin, and aluminium oxides. Scandinavian minerals containing smaller quantities of thoria are monazite (phosphate), gadolinite (silicate), euxenite (columbo-tantalate), samarskite (columbo-tantalate), and numerous other minerals in which the tervalent rare earth elements occur in considerable quantities. A mineral richer in thorium than any of the preceding is thorianite, which contains about 80 per cent, of thoria associated with the oxides of uranium and the tervalent rare earth elements. This mineral was discovered in Ceylon about 1904, but unfortunately is rather scarce, and therefore does not constitute an important source of thoria. In the earlier years of the incandescent gas mantle, that is about 1885, the world-supply of thoria was insufficient to meet the demand.

Subsequent to 1895, however, large quantities of monazite have been discovered, not in the form of the crystalline mineral, but in the form of monazite sand. These sands have been produced by the weathering of rocks which originally contained a very small percentage of monazite, and the subsequent washing away of the lighter materials produced. The monazite in the sands is associated with numerous other minerals, from which, however, it may be separated without much difficulty. Monazite is essentially an orthophosphate of the cerium group of rare earth elements, but it almost invariably contains some thorium, probably as phosphate. Deposits of monazite sand occur in various parts of the globe, notably in Brazil and in the native State of Travancore, India. These sands constitute almost the only source of the thorium used in the gas-mantle industry. The composition of Brazilian sand, as concentrated for exportation, varies between the following limits

ThO25-7 per cent.
P2O525-30 per cent
Ce2O325-35 per cent
20-30 per cent.
Y2O31-5 per cent
SiO21-4 per cent
Al2O3, Fe2O3, MnO, CaO, MgO, PbO, SnO2traces.

Concentrated Travancore sand is considerably richer in thorium than the above. The North American (North and South Carolina, Idaho) deposits of monazite sand are now of little or no commercial value.

Thorium is found in minute quantities in various igneous and sedimentary rocks.


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