On the Chemical Composition of the Outer Core

Abstract

The average composition of the Earth is assumed to be the same as that of the chondritic meteorites. This assumption is consistent with the present rate of heat production in the Earth. A core made up solely of iron-nickel is inconsistent with the chondritic model unless the ratio of the abundances of (Fe+ Mg)/Si is unity in the mantle. The condition (Fe + Mg)/Si = 1 requires that a major part of the mantle be made of material having the composition of pyroxene or pyroxene-garnet (eclogite). If the mantle is assumed to be made up of a 400 km shell of the composition of eclogite underlain by a material of dunite composition, the chondritic model requires a core of composition (Fe, Ni)1.6Si.The representative atomic number (Ẑ) has been introduced to characterize compounds at moderately high pressures. From asymptotic relations in the Thomas-Fermi-Dirac model, a basis is formed for the computation of Ẑ. Bullen's density distribution gives Ẑ for the outer core of about 22. The representative atomic number of Fe1.6Si is 22.8. The composition of the core deduced from equation of state arguments is thus consistent with the chondritic model of the Earth.The electrical conductivity of (Fe, Ni)1.6Si should be a third to a tenth that of pure iron for conditions obtained in the core. Such a conductivity is consistent with the requirements of the dynamo theory of the magnetic field.