8 - Trace element geochemistry: distribution of transition metals in the Earth's crust

Summary

– – although ionic radius and charge are important factors in determining (trace) element distributions, there are other energy factors that need to be considered.

P. Henderson, Inorganic Geochemistry (Pergamon Press, 1982), p. 134

Introduction

One outcome of interpreting transition metal geochemistry by crystal field theory is that the theory has enabled some of the basic concepts of geochemistry to be critically evaluated and defined more rigorously. In earlier chapters, crystal field theory was used to explain why some transition elements deviate from periodic crystal chemical and thermodynamic trends shown by other cations with similar charges and ionic radii. In this chapter, criteria for interpreting trace element geochemistry are examined. Examples are highlighted where fractionation patterns applicable to many elements sometimes deviate for transition metal ions. Crystal field effects are shown to be dominant factors influencing the distributions of several of the transition elements in crustal processes during the petrogenesis of igneous, sedimentary and metamorphic rocks.

Trace elements

The classification of chemical elements into major and minor or trace element categories is somewhat arbitrary. Thermodynamically, a minor element may be defined as one that is partitioned between coexisting phases in compliance with laws of dilute solutions, such as Henry's law, eq. (7.2b). In geochemical parlance, however, trace elements are usually categorized on the basis of abundance data. In this context, the mineral, rock or environment containing the chemical elements must be defined as well as the concentration boundary separating a major and trace element.