Molecular dynamics simulations of Y in silicate melts and implications for trace element partitioning
Molecular dynamics simulations of Y in silicate melts and implications for trace element partitioning
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DOI：
10.1016/j.chemgeo.2012.08.021
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发表时间：
2013-05
期刊：
影响因子：
3.9
通讯作者：
V. Haigis;M. Salanne;S. Simon;M. Wilke;S. Jahn
中科院分区：
文献类型：
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作者：
V. Haigis;M. Salanne;S. Simon;M. Wilke;S. Jahn
元素分配强烈地依赖于所涉及的相的组成和结构。在这项研究中，我们使用分子动力学模拟调查的本地环境Y作为一个示例性的微量元素在四个硅酸盐熔体具有不同的组合物，从而不同程度的聚合。基于这些结构的结果，我们提出了一个机制，解释了所观察到的Y和其他稀土元素之间的晶体和熔体或两个熔体之间的分配趋势。通过我们的计算方法，我们发现了熔体组成与Y配位以及Y-O键长之间的系统相关性，这一结果得到了与模拟熔体组成相同的玻璃的EXAFS光谱的证实。此外，我们的模拟揭示了Y作为第二近邻（本研究中的Ca）对网络修饰剂的亲和力以及避免网络形成剂（Si和Al）的趋势。这与Y（和其他稀土元素）在分配实验中通常优选解聚而不是聚合熔体的观察结果一致（参见，例如，施密特等人（2006年））。此外，我们使用的热力学积分的方法来计算吉布斯自由能，支配Y分配两个示例性熔体之间。这些更定量的结果也与观察到的分配趋势一致。
Element partitioning depends strongly on composition and structure of the involved phases. In this study, we use molecular dynamics simulations to investigate the local environment of Y as an exemplary trace element in four silicate melts with different compositions and thus varying degrees of polymerization. Based on these structural results, we propose a mechanism which explains the observed partitioning trends of Y and other rare-earth elements between crystals and melts or between two melts. With our computational approach, we found a systematic correlation between melt composition and Y coordination as well as Y―O bond lengths, a result which was corroborated by EXAFS spectroscopy on glasses with the same compositions as the simulated melts. Our simulations revealed, moreover, the affinity of Y for network modifiers as second-nearest neighbors (Ca in this study) and the tendency to avoid network formers (Si and Al). This is consistent with the observation that Y (and other rare-earth elements) in general prefer depolymerized to polymerized melts in partitioning experiments (see, e.g., Schmidt et al. (2006)). Furthermore, we used the method of thermodynamic integration to calculate the Gibbs free energy which governs Y partitioning between two exemplary melts. These more quantitative results, too, are in line with the observed partitioning trends.