Studies On Electrical Properties Of Minerals And Rocks Under Defined Thermodynamic Conditions

Electrical conductivity is an important geophysical parameter. Measurements of the electrical conductivity of minerals and rocks from the Earth's interior provide a powerful tool for probing physical and chemical properties and composition of the deep earth, and help us to interpret magnetotelluric data. A new measurement system for electrical conductivity in an YJ-3000t press fitted with a wedge-type cubic anvil was set up on the basis of the old one. A solartron 1260 impedance/Gain phase analyzer was used in the new system; Mo electrodes and a Mo shield were also used to keep oxygen fugacity close to the Mo-MoO2, which is similar to that of iron-wustite (IW). The high accuracy of electrical conductivity measurement could be reached with this new system, and oxygen fugacity could be well controlled.Electrical conductivities of quartz, olivine, dunite, Iherzolite, pyroxenite, and megaaugite were measured by virtue of the new system; whereas, the electrical conductivity of gabbro was measured by means of the old one.An interesting outgrowth of the present study is the measurement of the complex impedance of a series of quartz plates with different orientations. Measurements were made over the frequency range 0.1 tol06 Hz at 1-3 GPa and 600 -1600 K. The complex impedance plot displays one arc and one straight line at each temperature; they correspond to different conduction process or mechanism at different range of frequency, and have different relaxation time, respectively. The high-frequency arc represents bulk properties of the sample. The straight line is characteristic of diffusional processes at the sample-electrode interface. The phase angle displays a strong dependence on frequency; the impedance magnitude decreases with frequency. The electrical conductivity of quartz increases with temperature, but pressure has weak effect on the electrical conductivity. Conductivity mechanism of a -quartz is ionic, and alkali and hydrogen ions moving in channels parallel to the c-axis are the predominant current carriers. The electrical conductivity of quartz decreases with increasing angle to c axis at the same temperature and pressure due to decreasing of the channel size. There is no sudden change in the conductivity at the transition point.The complex impedance of gabbro was determined at 1-2 GPa and 593 - 1173 K, over a frequency range 12 to l5 Hz. The real part of the complex impedance decreases with frequency; the imaginary part of the complex impedance increases with frequency to the maximum, and then decreases with frequency; and phase angle decreases with frequency increasing. The grain interior arc occurs at highest frequency range. The electrical conductivity value of the gabbro is 1.77X l0-4S/m at 1.0 GPa and 893 K, which is 2-3 orders magnitude lower than that of high conductivity layer in the lower crust, so gabbro could not form the high conductivity layer.The polycrystalline olivine compacts were synthesized at 130 MPa and 1473 K for 2.5 hours. The complex electrical impedance of polycrystalline olivine compacts was determined at 3 GPa and 1273-1573 K, and at the fo2 of Mo-MoO2 over a frequency range 0.1 to 106Hz. The arcs representing grain interior conduction mechanisms appear on all complex impedance plots, and occur at high frequency rang and contract with temperature. However, those arcs representing grain boundary and occurring at low frequency range were observed only in a small number of experimental runs. The phase angles display a strong dependence on frequency; the impedance magnitudes decrease across the entire frequency spectrum. The activation enthalpies lie in the range of 1.03-2.11eV.The complex electrical properties of dunite were measured over the frequency range from 0.1-106Hz, at 1-3 GPa and 1282-1544K, and at the fo2 of Mo-MoO2. Complex plane plots show separate effects of grain interior and grain boundary conductivity. Grain interior transport controls the response above ~100 Hz, whereas grain boundary transport dominates between ~100 and 0.1 Hz. The electr...