QUANTITATIVE OLIVINE-DEFECT CHEMICAL-MODEL - INSIGHTS ON ELECTRICAL-CONDUCTION, DIFFUSION, AND THE ROLE OF FE CONTENT

By simultaneously solving the governing equations for defect formation and the relevant conservation laws, a general calculation of equilibrium concentrations of point defects in olivine as functions of temperature, oxygen fugacity (fO2), and Fe content is made. We present calculations both for olivine buffered by either pyroxene or magnesiowustite and for unbuffered olivine having fixed (Fe + Mg)/Si of 2 and Fe/(Fe + Mg) up to 0.1. The aim of this work is to better understand the mechanisms that control the transport processes of electrical conduction and diffusion. The following controlling mechanisms are consistent with calculated defect populations and experimental transport property data. Electrical conduction is dominated at low fO2 by electrons and at high fO2 by polaron hopping involving Fe3+ on Mg sites (Fe(Mg).) and Mg vacancies. Mg diffuses by a vacancy mechanism whereas O and Si more likely diffuse via interstitial mechanisms. We define the intrinsic-extrinsic division as the concentration of Fe at which the concentrations of other defects are perturbed; this division depends strongly on fO2 and temperature. Thus, even minute amounts of Fe (i.e. Fe/(Fe + Mg) = 1 ppm) may affect material properties at temperatures below 1000 K and fO2 above 10(-17) MPa, whereas at temperatures above 2000 K and fO2 below 10(-9) MPa, material properties may be unaffected up to Fe/(Fe + Mg) = 0.01. Although the energy parameters in the current model are probably not unique, calculated defect populations over wide ranges of temperature, fO2, buffer condition, and Fe content agree well with available experimental data.