Electrical conduction in the Mantle

2 Electrical conduction in the Mantle The magnitude and mechanism of electrical conductivity within the Earth have also been studied extensively. The radial distribution of electrical conductance in the Earth s interior may be estimated from studies of secular variations of the Earth s magnetic field. These measurements indicate a steep increase of electrical conductivity between depths of 400 and 1,000 km in the Mantle 

At a given pressure, the electrical conductivity o is related to absolute temperature by 

To account for these results for (Mg,Fe)Si03 perovskites, various charge transfer mechanisms have been proposed (Li and Jeanloz, 1990 Hirsch and Shankland, 1991 Sherman, 1991). Lattice defects permitting Fe3+ ions to exist in the perovskite structure give rise to oxygen — Fe and Fe2+ — Fe3+ charge transfer transitions, the latter being facilitated by the close proximity (279 pm) of the A sites (Fe2+) to the B sites (Fe3+) in the perovskite structure. The opacity of the hydrous phase D indicates that extensive electron delocalization may occur in its crystal structure. 

Chapter 9 describes how crystal field energy data obtained from measurements of electronic spectra of minerals at elevated pressures and temperatures may be applied to geophysical and geochemical features of the Mantle. 

Composition and mineralogy of the Mantle. The Earth s Mantle consists of Upper and Lower regions separated by the Transition Zone at depths between about 350 km and 650 km. Several phase changes occur in the Transition Zone in which common ferromagnesian silicates of the Upper Mantle, all containing Fe2+ ions in distorted six-coordinated sites and tetrahedrally coordinated Si, transform to dense oxide structures with cations occupying regular octahedral 

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