Simultaneous modeling of thermopower and electrical conduction in olivine

Abstract

 Measurements of conductivity and thermopower as a function of oxygen fugacity (ƒ_{O 2}) 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 ƒ_{O 2} dependence, and electrical conductivity exhibits approximately a ¹/₁₁ power dependence on ƒ_{O 2}. However, small polarons, considered to be the conducting defect in olivine at these temperatures, would produce a larger thermopower than observed, with a negative ƒ_{O 2} dependence, as well as ¹/₆ power dependence of conductivity on ƒ_{O 2}. At least one other conducting defect species must be invoked to explain the observed magnitude and ƒ_{O 2} 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.