5 - Condensation Sequences and the Geochemical Classification of the Elements

Summary

The Concept of Geochemical Classification

The Norwegian geochemist Victor Goldschmidt is the father of the notion of geochemical classifications of the chemical elements. Goldschmidt's (1954) posthumous work Geochemistry is still of great value. His basic aim was to divide the elements into groups which might be identified with the major divisions of the earth during its cooling history. He thought there might be three separate liquid phases, one metal, one silicate, and one primarily iron sulfide. These would be surrounded by a gaseous phase. He classified the elements from their association with, or preference for one or the other of these phases.

Let us begin with a consideration of the chemistry of meteorites and the earth. The earth may be divided into a core, a mantle and a crust. The chemistry of the core must be largely inferred, and this is essentially true for most of the mantle (Ringwood 1975, Pasteris 1984). The upper continental crust is relatively well sampled (Taylor and McLennan 1985), but it is not representative. Meteorites, on the other hand, have been repeatedly and thoroughly analyzed in the laboratory. Moreover, they are thought to be pieces of a broken-up planet, not unlike the earth (see, e.g., McSween 1987). Because of this they have been used to infer the chemistry of the earth as well as of much of the cosmos.