Thermoelectric power measurements on a silicate-perovskite and magnesiow¿stite assemblage at lower-mantle conditions [(Mg0.89Fe0.11)2SiO4 bulk composition at 48 GPa and temperatures up to 2500 K> yield a positive Seebeck coefficient, thus implying that the dominant charge carriers are positive and that electrons in the conduction band (intrinsic carriers) contribute little to the electrical conductivity. The present results suggest that electrical transport is likely dominated by extrinsic charge carriers throughout the entire mantle and, hence, the electrical conductivity can be a powerful tool for characterizing minor phases, impurities and defect states of the Earth's deep interior. ¿ American Geophysical Union 1994