Simultaneous modeling of thermopower and electrical conduction in olivine

Measurements of conductivity and thermopower as a function of oxygen fugacity (f(O2)) are used to derive a model for conduction in olivine. Thermopower at 1000-1200 degrees C is between 50 and 400 mu V/K and has a positive f(O2) dependence, and electrical conductivity exhibits approximately a 1/11 power dependence on f(O2). However, small polarons, considered to be the conducting defect in olivine at these temperatures, would produce a larger thermopower than observed, with a negative f(O2) dependence, as well as 1/6 power dependence of conductivity on f(O2) At least one other conducting defect species must be invoked to explain the observed magnitude and f(O2) 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 degrees C, as has been previously inferred from other data.