The Electrical Conductivity Of Mantle Minerals Under High Temperature And High Pressure

In this paper,by combing the high-pressure in-situ impedance spectroscopy and Diamond Anvil Cell technology,the electrical transport mechanism of olivine,siderite and goethite were investigated.The details experiments and analysis results are as follows:1.A new method for in-situ control of oxygen fugacity during the electrical measurement on Diamond Anvil Cell under high temperature and high pressure was established.We propose a new oxygen fugacity control technique that can be widely applied to the in situ measurement of conductivity in minerals and rocks with Diamond Anvil Cell devices.The magnetron sputtering technology,thin film micromachining technology and diamond anvil technology were combined to construct a new device and experimental method suitable for the oxygen fugacity of the in-situ control system under extreme conditions?high temperature and high pressure?.In the experiment,we improved the insulating gasket as the sample cavity,adding an oxygen fugacity buffer ring between the gasket insulation powder,which can control the oxygen atmosphere condition in the sample chamber,and can form a closed atmosphere by using the oxygen fugacity buffer material.Sputtering a metal and corresponding metal oxide film on the diamond anvil surface as a test electrode,it overcomes the defects caused by the manual wiring of the Diamond Anvil Cell.Depending on the needs of the experiment,the electrodes can be photolithographically formed into any desired configuration.Taking olivine as research object,we have obtained electrical conductivity of olivine at pressures and temperatures up to 4.0 GPa and 873 K under the control of oxygen fugacity conditions?Mo-MoO₂?.According to the Arrhenius relationship,some important physical parameters,such as activated enthalpy,activation energy and activation volume,were obtained,and the conduction mechanism in the olivine was determined to be a small polaron.The conduction mechanism of small polarons provides a reasonable explanation for the oxygen fugacity dependence of conductivity in olivine.2.Electrical transport properties of siderite under high pressureThe electrical data of the siderite under high pressure was obtained by high pressure in-situ impedance spectroscopy.The AC impedance spectrum of the siderite in the pressure range of 0.2⁴4.3 GPa was studied.Combined with the complex modulus spectrum,we found that the main conduction mechanism in the siderite is the grain conduction,and the grain boundary contribution is so small that it does not affect the electrical properties of the sample.The relationship between sample resistance,conductivity and relaxation frequency as a function of pressure were obtained by equivalent circuit fitting.As the pressure increases,the sample resistance decreases and the conductivity increases.This is because the lattice parameters of the sample shrink witn pressure increasing,the electron cloud overlap is deepened,and the transition distance of electrons between atoms decreases.The resistance,conductivity and relaxation frequency all occur discontinuously at 39.2 GPa,which is related to the spin-state transition of Fe²⁺from HS to LS in the siderite sample.3.Electrical transport properties of goethite under high pressureHigh-pressure in-situ impedance spectroscopy was performed on natural goethite,and impedance data in the range of 1.4²8.8 GPa was obtained.Comparing the Nyquist diagram and the complex modulus spectrum of goethite,it is confirmed that the grain boundary conduction in the goethite sample is negligible,mainly is grain conduction.The pressure has a certain modulation effect on the electrical parameters of the goethite sample.As the pressure increases,the resistance of the sample decreases and the conductivity increases.The relaxation frequency increases with pressure,and the Arrhenius relational analysis shows that the sample in charged and discharged process get faster.In the pressure range of 1.4²8.8 GPa,we found that the resistance,conductivity and relaxation frequency were discontinuous at 7.9 GPa and 14.4 GPa.The discontinuous change in the range of 7.9 GPa¹4.4 GPa is attributed to the structural phase transition.The?-FeOOH is transformed into the high-pressure phase?-FeOOH.The position of hydrogen ions and three iron ions in the two phases changes,which leads to the change of the electron transport path.