Shuangming Shan

The electrical conductivity of dry polycrystalline olivine compacts at high temperatures and pressures

Abstract The electrical conductivity of dry polycrystalline olivine compacts (hot-pressed and sintered pellets) was measured at pressures of 1.0-4.0 GPa, at temperatures of 1073-1423 K, and at different oxygen fugacities via the use of a YJ-3000t multi-anvil press. Oxygen fugacity was controlled successfully by means of five solid buffers: Fe3O4-Fe2O3, Ni-NiO, Fe-Fe3O4, Fe-FeO and Mo-MoO2. Within the selected frequency range of 102-106 Hz, the experimental results indicate that the grain interior conduction mechanism is characterized by a semi-circular curve on an impedance diagram. As a function of increasing pressure, the electrical conductivity of polycrystalline olivine compacts decreases, whereas the activation enthalpy and the temperature-independent pre-exponential factors increase slightly. The activation energy and activation volume of polycrystalline olivine compacts were determined to be 141.02±2.53 kJ/mol and 0.25±0.05 cm3/mol, respectively. At a pressure of 4.0 GPa, electrical conductivity was observed to increase as a function of increasing oxygen fugacity, and the relationship between electrical conductivity and oxygen fugacity can be described as log10 (σ) = (2.47±0.085) + (0.096±0.023) × log10 fO₂ + (-0.55±0.011)/T, which presents the exponential factor q (~0.096). Our observations demonstrate that the primary conduction mechanism for polycrystalline olivine compacts is a small polaron.

Electrical conductivity of albite at high temperatures and high pressures

Abstract The electrical conductivity of low albite has been measured using a complex impedance spectroscopic technique at 1.0-3.0 GPa and 773-1073 K in the frequency range of 10−1 to 106 Hz in a YJ-3000t multi-anvil press. Within this frequency range, the complex impedance plane displays a semi-circular arc that represents a grain interior conduction mechanism. The electrical conductivity of albite increases with increasing temperature, and the relationship between electrical conductivity and temperature fits the Arrhenius formula. Pressure has a weak effect on the electrical conductivity of albite in the experimental pressure-temperature (P-T) range in the present work. The pre-exponential factors decrease, and the activation enthalpy increases slightly with increasing pressure. The activation energy and activation volume of albite are 0.82 ± 0.04 eV and 1.45 ± 0.28 cm3/mol, respectively. Comparison with previous results with respect to albite indicates that our data are similar to previous data within the same temperature range. The dominant conduction mechanism in albite is suggested to be ionic conduction, where loosely bonded sodium cations, the dominant charge carriers, migrate into interstitial sites within the feldspar aluminosilicate framework. The Na diffusivity inferred from electrical conductivity of albite in this study using the Nernst-Einstein relation is consistent with that of previous studies on natural albite.

Application of the cBΩ model to the calculation of diffusion parameters of Si in silicates

Silicon diffusion in major mantle minerals plays an important role in understanding a number of physical and chemical processes in the Earths interior. Inspection of existing experimental data reveals linear compensation law between the preexponential factors and the activation energies for Si diffusion in various minerals by focusing on those of geophysical interest. On the basis of the observed compensation relationship, here we propose a thermodynamic model, the so-called cBΩ model that interconnects point defect parameters with the bulk properties to reproduce the Si self-diffusion coefficients in different rock-forming minerals. When the uncertainties are considered, the predicted results show that the temperature and pressure dependences of self-diffusion coefficients concur with existing experimental data and theoretical calculations in most cases.

Novel technique to control oxygen fugacity during high-pressure measurements of grain boundary conductivities of rocks.

This paper describes the development and application of a novel method for the measurement of grain boundary electrical conductivity of rock at high temperature and pressure. In this method, the metal electrodes, the corresponding metal shielding case, and the sleeves are altered in order to appropriately adjust and monitor the oxygen fugacity in a sample cavity in a high-pressure apparatus. As an example, a series of oxygen buffers including Fe(3)O(4) + Fe(2)O(3), Ni + NiO, Fe + Fe(3)O(4), Fe + FeO, and Mo + MoO(2) was selected and tested, and the oxygen fugacity was confirmed as adjusted during the process of electrical conductivity measurements. Application of this method provides a powerful means of restricting specific thermodynamic conditions at high temperature and pressure.

Thermodynamic calculations of Fe–Mg interdiffusion in (Mg,Fe)2SiO4 polymorphs and perovskite

In this study, we show that the temperature and pressure dependence of Fe–Mg interdiffusion in (Fe,Mg)2SiO4 polymorphs (olivine, wadsleyite, and ringwoodite) and perovskite can be successfully reproduced in terms of bulk elastic and expansivity data through a thermodynamic model (so-called cBΩ model) that interconnects point defect parameters with bulk properties. Under dry and wet conditions, our calculated Fe–Mg interdiffusion coefficients DcalcFe-Mg (particularly for anisotropic diffusivity in olivine), activation enthalpy hact, and activation volume υact over a wide range of geologically relevant temperatures (1000–2400 K) and pressures (0–100 GPa) are consistent with the experimental ones when the uncertainties are considered.

Phase boundary between perovskite and post-perovskite structures in MnGeO3 determined by in situ X-ray diffraction measurements using sintered diamond anvils

Abstract To determine the phase boundary between the perovskite and post-perovskite structures in MnGeO3, in situ X-ray observations were carried out at pressures of 57-68 GPa and temperatures of 1000-1900 K using the Kawai-type high-pressure apparatus equipped with sintered diamond anvils interfaced with synchrotron radiation. The phase boundary was determined to be P (GPa) = 39.2 + 0.013T (K) based on Tsuchiya’s (2003) gold pressure scale. The Clapeyron slope, dP/dT, of 13(+12/-5) MPa/K, determined in the present study is larger that of MgGeO3 and MgSiO3.

P-V-T relations of γ-Ca3(PO4)2 tuite determined by in situ X-ray diffraction in a large-volume high-pressure apparatus

Abstract Tuite, γ-Ca3(PO4)2, is regarded as an important phosphate mineral in the deep mantle playing a crucial role as a host for rare earth elements, large ion lithophile elements, and phosphorus. In this study we report the thermoelastic properties of synthetic γ-Ca3(PO4)2 at simultaneously high pressures and temperatures of up to 35.4 GPa and 1300 K, respectively, as determined by means of in situ energydispersive X‑ray diffraction in a large-volume multi-anvil apparatus. The pressure-volume-temperature data obtained for γ-Ca3(PO4)2 were fitted by the high-temperature Birch-Murnaghan equation of state to yield V2 = 447.4(4) Å3, KT0 = 100.8(18) GPa, K′T0 = 5.74(13), (∂KT/∂T)P = -0.020(1) GPa/K, and αT = 3.26(18) × 10-5 + 1.76(24) × 10-8 T. In addition, fitting the present data to the Mie-Grüneisen-Debye equation of state gives γ0 = 1.35(6), Θ0 = 944(136) K, and q = 0.37(29). Based on the thermoelastic properties obtained in our study, the density profiles of γ-Ca3(PO4)2 tuite along typical cold and hot slab geotherms were calculated and are compared with those of the coexisting silicate minerals in subducting mid-ocean ridge basalt.

Experimental measurement of the electrical of the electrical conductivity of single crystal olivine at high temperature and high pressure under different oxygen fugacities

Abstract At 1.0–4.0 GPa and 11213–1473 K and under oxygen fugacity-controlled conditions (Bi + NiO, Fe + Fe3O4, Fe + FeO and Mo + MoO2 buffers), a YJ-3000t Model six-anvil solid high-pressure apparatus and a Sarltron-1260 Impedance/Gain-Phase analyzer were employed to conduct an in situ measurement of the electrical conductivity of single crystal olivine. Experimental results showed that: (1) within the range of experimentally selected frequencies (103–106 Hz), the electrical conductivity of the sample is of great dependence on the frequency; (2) with the rise of temperature (T); the electrical conductivity (σ) will increase, and the Arrenhius linear relationship is established between 1gσ and 1/T; (3) under the control of oxygen buffer Fe + Fe3O4, with the rise of pressure, the electrical conductivity tends to decrease whereas the activation enthalpy and independent-of-temperature preexponential factor tend to increase, with the activation energy and activation volume of the sample estimated at (1.25 + 0...

Modeling H, Na, and K diffusion in plagioclase feldspar by relating point defect parameters to bulk properties

Hydrogen and alkali ion diffusion in plagioclase feldspars is important to study the evolution of the crust and the kinetics of exsolution and ion-exchange reactions in feldspars. Using the available PVT equation of state of feldspars, we show that the diffusivities of H and alkali in plagioclase feldspars as a function of temperature can be successfully reproduced in terms of the bulk elastic and expansivity data through a thermodynamic model that interconnects point defect parameters with bulk properties. Our calculated diffusion coefficients of H, Na, and K well agree with experimental ones when uncertainties are considered. Additional point defect parameters such as activation enthalpy, activation entropy, and activation volume are also predicted. Furthermore, the electrical conductivity of feldspars inferred from our predicted diffusivities of H, Na, and K through the Nernst–Einstein equation is compared with previous experimental data.

Compressibility of pyrochlore-type MgZrSi2O7 determined by in situ X-ray diffraction in a large-volume high pressure apparatus

The pyrochlore-type MgZrSi2O7 has been investigated by in situ synchrotron energy-dispersive X-ray diffraction measurements up 24.3 GPa at room temperature in a multianvil high pressure apparatus. The pressure has been calculated using an internal gold metal pressure calibrant. No phase transformation was observed in this study. A third-order Birch–Murnaghan equation of state was used to fit the pressure–volume data. The fitting gives a volume of V 0=802.8±0.2 Å3, an isothermal bulk modulus of K T =249±12 GPa, and first pressure derivative of K′ T =3.2±1.3. If K′ T is fixed as 4, K T is obtained as 241.8±2.6 GPa. Compared with zirconate or titanate pyrochlores, MgZrSi2O7 shows a smaller compressibility due to a larger cation radii ratio (r Mg,  Zr /r Si).