This paper reports detailed measurements of electrical conductivity σ and thermoelectric effect S in the mineral olivine and in synthetic forsterite as functions of temperature in the range from 1000° to 1500°C and oxygen partial pressure in the range from 10−10 to 104 Pa. The two most striking observations are strong conductivity anisotropy in forsterite and a sign change in S in olivine at 1390°C. These results are interpreted to show that both materials have mixed ionic and extrinsic electronic conduction under these conditions. On the basis of these interpretations, we infer that forsterite conductivity is dominated by electronic conduction in the a and b directions and probably by movement involving magnesium vacancies in the c direction, where far higher, PO2‐independent conductivity is observed. Olivine appears to show mixed conduction under all the circumstances observed; at low temperatures, electron holes dominate but are superseded by magnesium vacancies at high temperatures.
In order to extend the useful temperature range of interpretation of olivine electrical conductivity σ we have used the nonlinear iterative Marquardt technique to fit experimental data over the range 720°–1500°C to the parametric form σ = σ1e−A1/kT + σ2e−A2/kT, where k is Boltzmann's constant and T is absolute temperature. The model describes conduction by migration of two different thermally activated defect populations with activation energies A1 and A2, and preexponential terms σ1 and σ2 that depend on number of charge carriers and their mobility and that may be different for each crystallographic direction. A combined interpretation of recent high (San Carlos olivine) and low (Jackson County dunite) temperature measurements has been made that demonstrates that a single activation energy A1 for all three crystallographic directions adequately fits the data. The parametric fits show that the high‐temperature conduction mechanism has far greater anisotropy than the low‐temperature mechanism, consistent with previous assignments to ionic and electronic conduction, respectively. The geometric mean of the conductivity in the three directions is approximately
–¯=102.402e‐1.60eV/kT+109.17e‐4.25eV/kT
S/m and is presented as a model for isotropic olivine, SO2, appropriate from 720°C to above 1500°C, at oxygen fugacities near the center of the olivine stability field. It is observed that the magnitudes of σ1 for the three crystal directions are similar to the ratios of the inter‐ionic distances between the M1 magnesium sites in olivine, to within 5%, consistent with Fe3+ preferring the M1 site below 1200°C.
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