Simultaneous Modeling of Thermopower and Electrical Conduction in Olivine

Measurements of conductivity and thermopower as a function of oxygen fugacity (fO2) are used to derive a model for conduction in olivine. Thermopower at 1000– 1200◦C is between 50 and 400 μV/K and has a positive fO2 dependence, and electrical conductivity exhibits approximately a /11 power dependence on fO2 . However, small polarons, considered to be the conducting defect in olivine at these temperatures, would produce a larger thermopower than observed, with a negative fO2 dependence, as well as a /6 power dependence of conductivity on fO2 . At least one other conducting defect species must be invoked to explain the observed magnitude and fO2 dependence of thermopower. An electron/polaron model cannot be made to fit the conductivity and thermopower data well, but a polaron/magnesium vacancy model fits the data if a constant polaron or magnesium vacancy term is included. Concentrations from our fits are consistent with predictions from theoretical models, and our analysis predicts a transition from polaron dominance in conduction to magnesium vacancy dominance at around 1300◦C, as has been previously inferred from other data.