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numbers expressing the average content of the chemical elements in the earth’s crust, the hydrosphere, the earth as a whole, celestial bodies, and other geochemical and cosmochemical systems.
A distinction is made between weight clarkes (expressed in percent; in g per ton; or in g per g) and atomic clarkes (expressed as a percentage of the number of atoms). A summary of the data on the chemical composition of the various rocks composing the earth’s crust to a depth of 16 km was carried out for the first time by the American scientist F. W. Clarke in 1889. The figures he obtained for the percentage content of the chemical elements making up the earth’s crust were refined somewhat by A. E. Fersman. Upon Fersman’s proposal the figures were called Clarke numbers, or clarkes.
The average content of the elements in the earth’s crust, according to the current understanding of the crust as the upper layers of the planet above the Moho (Mohorovičič discontinuity), were calculated by A. P. Vinogradov in 1962, by the American scientist S. R. Taylor in 1964, and by the German scientist K. H. Wedepohl in 1967 (see Table 1). There is a predominance of elements of low atomic number; 15 of the most widely found elements (the clarkes of which are over 100 g per ton) have atomic numbers under 26 (iron). Even-numbered elements make up 87 percent of the mass of the earth’s crust; those with odd numbers make up the remaining 13 percent. The average chemical composition of the earth as a whole has been calculated on the basis of data obtained on the composition of meteorites.
|Table 1. Average content of elements in the earth’s crust (g per ton)|
|Atomic number||Element||According to A. P. Vinogradov (1962)||According to K. H. Wedepohl (1967)|
Since clarkes are used as a standard for comparing increased and reduced concentrations of chemical elements in mineral deposits, rocks, or entire regions, a knowledge of them is important in prospecting and in the industrial evaluation of mineral deposits. Clarkes also make it possible to judge a disruption in the usual ratios between similar elements (chlorine-bromine, niobium-tantalum), and thereby to indicate the various physico-chemical factors that have disrupted the equilibrium ratios.
In the processes of element migration, clarkes serve as a quantitative index of the element’s concentration.
REFERENCESTaylor, S. R. “Abundance of Chemical Elements in the Continental Crust: A New Table.” Geochimica et Cosmochimica Acta, 1964, vol. 28, No. 8, pp. 1273–85.
Wedepohl, K. H. Geochemie. Berlin, 1967. (Sammlung Göschen, vols. 1224–1224a/1224b.)
V. V. SHCHERBINA