Abstract Iron (Fe) is present in terrestrial melts and at all depths inside the Earth. How Fe in its varying oxidation and spin states influences the properties of silicate melts is of critical importance to the understanding of the chemical evolution of our planet. Here, we report the results of first-principles molecular dynamics simulations of molten Fe-bearing MgSiO3 over a wide pressure range covering the entire mantle. Our results suggest that the structural properties of the host melt, such as the average bond length and coordination in Mg–O and Si–O do not differ much when compared with the pure melt. More importantly, they show that the local Fe–O structure is more sensitive to the spin state (high-spin, HS and low-spin, LS) of iron than to its valence state (Fe2+ and Fe3+). For iso-valence configurations, the average Fe–O bond length and coordination number differ by more than 10% and ∼30%, respectively, between the HS and LS states. In comparison, the corresponding differences between Fe2+ and Fe3+ for iso-spin configurations are within 5 and 15%, respectively. Ferrous iron shows lower average oxygen coordination numbers of ∼3.8 for HS and ∼3.3 for LS compared to the corresponding numbers of ∼4.1 and ∼3.7 for ferric iron at 0 GPa and 3000 K. As pressure increases, the coordination gap between the ferrous and ferric iron closes for HS but persists for LS. Our analysis of the proportions of non-bridging and bridging oxygens and the rates of bond breaking/formation events suggests an equivalent role of the ferrous and ferric iron in terms of their network forming ability. The predicted structural behavior of iron in its different oxidation states is generally consistent with the experimental inferences for MgO–FeO–SiO2 melts. Unlike other ferrosilicate compositions for which the experimental data suggest that Fe3+ increases and Fe2+ decreases the viscosity of the melt, the ferrous and ferric iron, due to their structural equivalence, are likely to have a similar influence on the dynamical behavior of deep mantle iron-bearing MgSiO3 melts.

中文翻译：

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Fe在MgSiO3熔体中的价态和自旋的影响：来自第一性原理分子动力学模拟的结构洞察

摘要 铁 (Fe) 存在于地球熔体中和地球内部的所有深处。处于不同氧化和自旋状态的 Fe 如何影响硅酸盐熔体的性质，对于理解我们星球的化学演化至关重要。在这里，我们报告了在覆盖整个地幔的宽压力范围内对含铁的 MgSiO3 进行第一性原理分子动力学模拟的结果。我们的结果表明，与纯熔体相比，主体熔体的结构特性，例如 Mg-O 和 Si-O 中的平均键长和配位并没有太大差异。更重要的是，他们表明局部 Fe-O 结构对铁的自旋态（高自旋，HS 和低自旋，LS）比对其价态（Fe2+ 和 Fe3+）更敏感。对于等价构型，在 HS 和 LS 状态之间，平均 Fe-O 键长和配位数分别相差超过 10% 和 ~30%。相比之下，同旋构型的 Fe2+ 和 Fe3+ 之间的相应差异分别在 5% 和 15% 以内。与三价铁在 0 GPa 和 3000 K 下的~4.1 和~3.7 的相应数字相比，亚铁的平均氧配位数较低，HS 为~3.8，LS 为~3.3。三价铁对 HS 关闭，但对 LS 持续。我们对非桥连和桥连氧的比例以及键断裂/形成事件的速率的分析表明，亚铁和三价铁在其网络形成能力方面具有相同的作用。不同氧化态的铁的预测结构行为通常与 MgO-FeO-SiO2 熔体的实验推论一致。与实验数据表明 Fe3+ 增加而 Fe2+ 降低熔体粘度的其他铁硅酸盐成分不同，亚铁和三价铁由于它们的结构等效性，可能对深地幔铁的动力学行为产生类似的影响。含 MgSiO3 熔体。