Some observations on the distribution of thallium, cadmium and bismuth in silicate rocks and the significance of covalency on their degree of association with other elements

Abstract

Cd, Tl and Bi have been estimated in forty-four rock samples, using the anion exchange—spectrochemical procedure described by Brooks et al. (1960). Estimates of the crustal abundances of these elements have been made on the basis of two parts granite (plus compositional equivalent) and one part basalt (plus gabbro and diabase), and are as follows: Cd (0·08 p.p.m.), Tl (0·7 p.p.m.) and Bi (0·17 p.p.m.). The Cd value is lower than previous estimates. The Tl value is also lower than other estimates but as it is based on quite extensive data (recent data of Russian and Japanese workers, plus data from this paper) it is suggested with considerable confidence. The Rb-Tl association is considered in some detail. The association of these two elements is in certain respects closer than that of the classical pair K-Rb. The presence of a significant degree of covalency in the Tl—O bond may slightly but distinctly influence the Rb-Tl association (see Taylor and Heier, 1960). A high degree of covalency is evidently present in the Cd—0 bond and is the probable reason why a Cd-Ca association (radii of Cd 2+ and Ca 2+ almost identical) is virtually non-existent. The significance of the presence of a high degree of covalency in the metal-oxygen bonds of certain other elements is also discussed. Possible high crustal enrichment of Tl and Bi, but not Cd, is noted.