Clarkes


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Clarkes

 

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 numberElementAccording to A. P. Vinogradov (1962)According to K. H. Wedepohl (1967)
1H 700
2He (0.003)
3Li3230
4Be3.82
5B129
6C200320
7N1920
8O470,000472,500
9F660720
10Ne  
11Na25,00024,500
12Mg18,70013,900
13Al80,50078,300
14Si295,000305,400
15P930810
16S370310
17Cl170320
18Ar (0.04)
19K25,00028,200
20Ca33,00028,700
21Sc1014
22Ti4,5004,700
23V9095
24Cr8370
25Mn1,000690
26Fe46,50035,400
27Co1812
28Ni5844
29Cu4730
30Zn8360
31Ga1917
32Ge1.41.3
33As1.71.7
34Se0.050.09
35Br2.12.9
36Kr  
37Rb150120
38Sr340290
39Y2934
40Zr170160
41Nb2020
42Mo1.11
43Tc  
44Ru (0.001)
45Rh (0.001)
46Pd0.0130.01
47Ag0.070.06
48Cd0.13 
49In0.250.07
50Sn2.53
51Sb0.5 
52Te0.001(0.002)
53I0.4 
54Xe  
55Cs3.72.7
56Ba650590
57La4944
58Ce7075
59Pr97.6
60Nd3730
61Pm  
62Sm88.6
63Eu1.31.4
66Dy56.1
67Ho1.71.8
68Er3.33.4
69Tu0.27 
70Yb3.33.4
71Lu0.81.1
72Hf13
73Ta2.53.4
74W1.31.3
75Re0.0007(0.001)
76Os(0.001) 
77Ir (0.001)
78Pt (0.005)
79Au0.00430.004
80Hg0.0830.08
81TI11.3
82Pb1615
83Bi0.0090.2
84Po  
85At  
86Rn  
87Fr  
88Ra  
89Ac  
90Th1311
91Pa  
92U2.53.5

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.

REFERENCES

Taylor, 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

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