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首页> 外文期刊>The American mineralogist >Structure, thermodynamic, and transport properties of molten Mg2SiO4: Molecular dynamics simulations and model EOS
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Structure, thermodynamic, and transport properties of molten Mg2SiO4: Molecular dynamics simulations and model EOS

机译：熔融Mg2SiO4的结构，热力学和传输性质：分子动力学模拟和EOS模型

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Molecular dynamics simulations have been used to study the structure, equation of state (EOS), self-diffusion, and shear viscosity of molten Mg2SiO4 for pressures and temperatures in the range 2.5–110 GPa and 2100–5060 K, respectively. The transferable pair-potential parameters of Matsui (1998) for the system Na2O-CaO-MgO-Al2O-SiO2 have been used accounting for Coulomb, Born, and van der Waals forces. Simulations have been carried out in the microcanonical (NEV) ensemble at 63 state points along 12 isochores spanning the density range 2754–4500 kg/m3. Thermodynamic properties including the isochoric heat capacity, isobaric expansivity, isothermal compressibility, thermal pressure, and the Grüneisen parameter (γ) are computed directly from MD results. A density crossover between molten Mg2SiO4 and forsterite crystals occurs at ~15 GPa at 2100 K. We find the Grüneisen parameter to be a function of temperature (T), increasing with increasing T at low density (ρ < 3400 kg/m3) but decreasing as T rises at high density (ρ > 3400 kg/m3); hence, the integrated form of the Mie-Grüneisen EOS is only approximately valid for liquid Mg2SiO4 since γ varies by ~20% over the T range along an isochore. Radial distribution functions for all atoms around all other atoms were used to generate coordination statistics as a function of pressure (P) and T. Oxygen about Si coordination increases from fourfold coordination at low pressure to sixfold at higher pressure; the abundance of distorted trigonal bipyramidal fivefold polyhedra, Si(V) maximizes at 30 GPa at 3500 K. Interestingly, O about O increases to a maximum of 13 at low P before decreasing with increasing pressure to ~10. The mean coordination number (CN) of Si around oxygen increases from 1.2 to 1.5 consistent with an increasing abundance of Si2O7 dimers as pressure increases. Self-diffusion of Mg, Si, and O was calculated at each state point giving activation energies of 67, 79, and 76 kJ/mol and activation volumes of 1.42, 1.10, and 1.32 cm3/mol, respectively. Shear viscosity of the liquid calculated at 12 state points using the Green-Kubo formulation provides an excellent Arrhenian fit. Viscosity varies by a factor of ~20 (1.5 × 10–3 Pa s to 0.03 Pa s) from 1 to 100 GPa. The validity of the Stokes-Einstein and Eyring expressions for atom mobility and shear viscosity is examined in detail. Characteristic lengths for atom mobility are consistent with ionic radii to within a factor of ~1.5–2 for all atoms. An equation of state and thermodynamic model for Mg2SiO4 liquid is developed consistent with the fundamental measure functional theory of Rosenfeld and Tarazona (1998). Our model reproduces the E-P-V-T relations and the derived thermodynamic properties obtained from the MD simulations to within the reported uncertainty.
机译：分子动力学模拟已用于研究压力和温度分别在2.5–110 GPa和2100–5060 K范围内的熔融Mg2SiO4的结构，状态方程（EOS），自扩散和剪切粘度。 Na2O-CaO-MgO-Al2O-SiO2系统的Matsui（1998）的可转移对势参数已用于解释库仑力，玻恩力和范德华力。在微经典（NEV）集合中，沿着密度范围2754–4500 kg / m3的12个等时线在63个状态点进行了模拟。直接从MD结果中计算出的热力学性质包括等容热容，等压膨胀性，等温可压缩性，热压和Grüneisen参数（γ）。 Mg2SiO4和镁橄榄石晶体在2100 K处约15 GPa时发生密度交叉。我们发现Grüneisen参数是温度（T）的函数，在低密度（ρ<3400 kg / m3）下，T随温度的增加而增加，但随温度的降低而减小当T以高密度（ρ> 3400 kg / m3）上升时;因此，Mie-GrüneisenEOS的积分形式仅对液态Mg2SiO4近似有效，因为γ沿着等时线在T范围内变化了〜20％。围绕所有其他原子的所有原子的径向分布函数用于生成作为压力（P）和T的函数的配位统计。关于Si配位的氧从低压下的四倍配位增加到高压下的六倍;变形的三角双锥体五重多面体Si（V）的丰度在3500 K在30 GPa时达到最大值。有趣的是，在低P下，O大约O最高增加到13，然后随着压力增加到〜10而减小。 Si在氧周围的平均配位数（CN）从1.2增加到1.5，这与Si2O7二聚体随着压力的增加而增加。在每个状态点计算Mg，Si和O的自扩散，分别得到67、79和76 kJ / mol的活化能以及1.42、1.10和1.32 cm3 / mol的活化体积。使用Green-Kubo配方在12个状态点计算的液体的剪切粘度提供了出色的阿累尼西拟合。粘度在1至100 GPa之间变化约20倍（1.5×10–3 Pa s至0.03 Pa s）。详细检查了Stokes-Einstein和Eyring表达式对于原子迁移率和剪切粘度的有效性。对于所有原子，原子迁移率的特征长度与离子半径一致，约为1.5-2。根据Rosenfeld和Tarazona（1998）的基本度量函数理论，建立了Mg2SiO4液体的状态方程和热力学模型。我们的模型再现了E-P-V-T关系以及从MD模拟获得的导出的热力学特性，以达到所报告的不确定性。

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