Compressed Potassium: A Siderophile Element

Abstract

In recent years potassium has been the subject of considerable controversy regarding its possible presence in the earth’s outer core. Two questions must be answered in order to resolve this riddle: (1) Is potassium miscible in candidate iron melts under conditions prevalent in the deep interior of the earth?, and (2) Was the mechanism of core formation such that potassium was initially present in the core? The second question is by far the more difficult one and it must await further developments in our understanding of cosmochemistry, planetary accretion and the history of the internal dynamics of the earth. The first question has been the subject of various experimental and theoretical investigations. Unfortunately, the experiments were done at low pressures and their results are in conflict with each other. The same comment is applicable to the various theoretical cosmochemical arguments. Bukowinski [1] summarized the relevant literature and argued that low pressure data is probably not useful for making predictions about the high pressure behavior of potassium. In fact, a detailed study of the electronic structure and equation of state of potassium has shown the latter to have properties typical of a transition metal at pressures well below those in the earth’s core [1]. It was also shown that, due to electronic collapses, it is impossible to predict the correct high pressure properties of potassium through extrapolations of low pressure data.