ELECTRICAL-CONDUCTION AND POLARON MOBILITY IN FE-BEARING OLIVINE

Electrical conductivity sigma of Mg2(1-Y)Fe2YSiO4 olivines was measure d as a function of Fe content (Y = 0.09, 0.11, 0.18, 0.34) at temperat ures between 1423-1573 K and at oxygen fugacities slightly below the w ustite-magnetite buffer. Conductivity was proportional to X(Fe)1.8 for the conditions examined. In accord with current and previous experime nts and defect models at these conditions, conductivity is dominated b y small polaron hopping of holes from Fe3+ to Fe2+ on the Mg sublattic e. From quantitative equilibrium defect chemistry calculations, we fin d that the Fe3+ concentration is proportional to X(Fe)0.73. From measu red conductivity and calculated defect concentrations, the hopping mob ility is proportional to X(Fe)1.1 and is in the range (1-6) x 10(-4) c m2 V-1 s-1. This 1.1 power dependence on Fe content for mobility is cl ose to the value of approximately 1.0 independently deduced from polar on hopping theory for mixed valence solid solution systems. Therefore, small polaron conduction self-consistently explains the observed 1.8 power dependence of sigma on Fe content. Using the measured dependence of conductivity on Fe content, a change in composition from Fa10 to F a12 produces a conductivity increase by a factor of 1.39; near 1473 K such a conductivity change could be misinterpreted as an equivalent te mperature increase of 52 K in the upper mantle.