MgO Miscibility in Liquid Iron

Leslie Insixiengmay, Lars Stixrude
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Abstract

We explore phase equilbria on the MgO-Fe join as a prototype of lithophile-core interaction in terrestrial planets. Our simulations, based on density functional theory, are based on a two-phase method: fluids of initially pure MgO and Fe compositions are allowed to establish a dynamic equilbrium across a near-planar interface. Methods for analyzing the composition and other properties of the two coexisting phases show that MgO behaves as a component, with indistinguishable Mg and O concentrations in Fe-rich and oxide-rich phases. The phase diagram is well described as that of a symmetric regular solution, a picture confirmed by independent one-phase determinations of the enthalpy, entropy, and volume of mixing. The critical temperature, above which there is complete miscibility across the MgO-Fe join is 7000 K at 68 GPa, and 9000 K and 172 GPa. The rate of MgO exsolution from the Fe-rich liquid on cooling is similar to that found in previous experimental studies, and is too small to drive a dynamo.
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氧化镁在液态铁中的混溶性
我们探索了氧化镁-铁连接处的相平衡,以此作为陆地行星中嗜岩者-岩核相互作用的原型。我们的模拟以密度泛函理论为基础,采用两相法:让初始成分为纯氧化镁和铁的流体在近平面界面上建立动态平衡。分析这两种共存相的成分和其他性质的方法表明,氧化镁是一种成分,在富含铁和富含氧化物的相中,镁和氧的浓度无法区分。相图很好地描述了对称正则溶液的相图,对焓、熵和混合体积的独立单相测定也证实了这一点。在 68 GPa 时,临界温度为 7000 K;在 172 GPa 时,临界温度为 9000 K。冷却时富铁液体中氧化镁的溶解速率与以前的实验研究发现的速率相似,而且小得不足以驱动发电机。
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