UCL Discovery

Abstract

The Earth's core is largely composed of iron (Fe), alloyed with less dense elements such as sulphur, silicon and/or oxygen. The phase relations and physical properties of both solid and liquid Fe alloys are therefore of great geophysical importance. Over the past fifty years the properties of Fe and its alloys have been extensively studied experimentally. However, achieving the extreme pressures and temperatures found in the core provide a major experimental challenge, and there are still considerable discrepancies in the results obtained by using different experimental techniques. In the past fifteen years quantum mechanical techniques have been applied to predict the properties of Fe. First principles methods used to study Fe now enable us to conclude: (1) that pure Fe adopts an hexagonal close packed structure under core conditions and melts at similar to6200 K at 330 GPa, (ii) that thermodynamic equilibrium and observed seismic data are satisfied by a liquid Fe alloy outer core with a composition of similar to10 mole% S (or Si) and 8 mole% 0 crystallising at 5500 K to give an Fe alloy inner core with similar to8 mole% S (or Si) and 0.2 mole % 0, and (iii) that with such concentrations of S (or Si), an Fe alloy might adopt a body centred cubic structure in all or part of the inner core. In the future the roles of Ni, C, H and K in the core need to be studied, and techniques to predict the transport and theological proper-ties of Fe alloys need to be developed.