IMPEDANCE SPECTROSCOPY OF SINGLE AND POLYCRYSTALLINE OLIVINE - EVIDENCE FOR GRAIN-BOUNDARY TRANSPORT

The impedance spectra of single and polycrystalline olivine display tw o and three impedance arcs, respectively. Impedance spectra of single crystal olivine, polycrystalline olivine compacts, and natural dunite are compared to deduce the causes of the different impedance arcs. Var iation of sample dimensions and use of three- and four-electrode confi gurations aid in the interpretation. The resistance of the two highest frequency mechanisms varies directly with the length to area ratio (l /A) of the sample. Experiments using the four-electrode configuration confirm that the lowest frequency impedance arc is caused by processes at the sample-electrode interface. In both single and polycrystalline samples the highest frequency mechanism is interpreted as bulk (grain interior) conduction, and the lowest frequency mechanism is attribute d to sample-electrode interface effects. In the polycrystalline sample s, the intermediate frequency mechanism is interpreted as the grain bo undary conduction mechanism. The resistances of the grain interior and grain boundary mechanisms add in a series manner.