Grain size effect on the electrical conductivity of clinopyroxene

Complex impedance spectra of polycrystalline samples (with grain size fractions similar to 5-63, 63-160 and 160-250 mu m) and a single crystal sample (with orientation parallel to ), prepared from a natural megacryst augite, were measured in a piston cylinder apparatus at 10 kbar and 500-1,000A degrees C and with a Solartron 1260 Impedance/Gain Phase analyzer over a frequency range of 0.1-10(6) Hz. The main charge carriers are attributed to small polarons, and the activation enthalpy is 83 +/- A 3 to 90 +/- A 3 kJ/mol. The measured electrical conductivity shows no difference between the polycrystalline and single crystal samples, suggesting independence of electrical conductivity on grain size given a change above similar to 5 mu m. The electrical conductivity of augite is much higher than that of olivine, indicating that, if regionally enriched, augites may lead to zones of high electrical conductivity and electrical anisotropy in the deep lithosphere.