Experimentally, silica activity (aSiO2) has been shown to have an effect on Mg diffusion in forsterite, but no fully satisfactory mechanism has yet been proposed. We calculated the effects of aSiO2 and aluminium content (the main contaminant in some recent experimental studies), and their co-effect, on Mg diffusion in forsterite, using thermodynamic minimisations of defect formation energies [calculated using density functional theory (DFT)] and a Monte-Carlo diffusion model. These two variables, in isolation, do not appreciably change the defect concentrations of forsterite and thus do not affect the diffusivity of Mg. However, when elevated together, they cause large increases in the Mg vacancy content and thus can increase the Mg diffusivity by one to six orders of magnitude depending on temperature, with little pressure dependence. This effect is largely independent of Al2O3 concentration above ~ 1 wt. ppm, and thus, for all practical purposes, should occur wherever forsterite is in the presence of enstatite. It is also largely dependent upon configurational entropy and is thus highly sensitive to the chemistry of the crystal. A low concentration of structurally bound hydroxyl groups at low temperatures (1000 K) suppresses this effect in pure forsterite, but it is likely robust in the presence of water either when alternative water sinks (such as Ti or Fe) are present, or at high temperatures (> 1500 K). This effect is also robust in the presence of ferrous iron (or other substitutional Mg defects) at all temperatures. Fe2O3 can operate like Al2O3 in this reaction and should enhance its effect. These findings explain the experimentally observed dependency of Mg diffusion of aSiO2, and elucidate how chemical activity variations in both experiments and natural settings could affect not only the diffusivity of Mg in forsterite, but of olivine-hosted cations in general.

中文翻译：

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解释镁橄榄石中 M 位扩散对二氧化硅活性的依赖性：密度泛函理论方法

实验证明，二氧化硅活性 (aSiO2) 对镁橄榄石中的 Mg 扩散有影响，但尚未提出完全令人满意的机制。我们使用缺陷形成能量的热力学最小化计算了 aSiO2 和铝含量（最近一些实验研究中的主要污染物）及其共同作用对镁橄榄石中的扩散的影响 [使用密度泛函理论 (DFT) 计算] 和蒙特卡罗扩散模型。孤立地，这两个变量不会明显改变镁橄榄石的缺陷浓度，因此不会影响 Mg 的扩散率。然而，当它们一起升高时，它们会导致 Mg 空位含量大幅增加，因此可以根据温度将 Mg 扩散率增加 1 到 6 个数量级，而压力依赖性很小。这种效应在很大程度上与高于约 1 重量份的 Al2O3 浓度无关。ppm，因此，出于所有实际目的，应该出现在镁橄榄石存在顽火石的任何地方。它还很大程度上取决于构型熵，因此对晶体的化学性质高度敏感。在低温 (1000 K) 下，低浓度的结构结合羟基会抑制纯镁橄榄石中的这种效应，但当存在替代水汇（如 Ti 或 Fe）时，或在高浓度条件下，它可能在有水的情况下很稳定。温度 (> 1500 K)。在所有温度下，在亚铁（或其他替代镁缺陷）存在的情况下，这种影响也很明显。在该反应中，Fe2O3 可以像 Al2O3 一样发挥作用，并应增强其效果。