Benisek Artur, Dachs Edgar
Chemistry and Physics of Materials, University of Salzburg, Jakob-Haringer-Str. 2a, 5020 Salzburg, Austria.
Phys Chem Miner. 2020;47(3):15. doi: 10.1007/s00269-020-01085-8. Epub 2020 Feb 17.
Calculations using the density-functional theory (DFT) in combination with the single defect method were carried out to determine the heat of mixing behaviour of mineral solid solution phases. The accuracy of this method was tested on the halite-sylvite (NaCl-KCl) binary, pyrope-grossular garnets (MgAlSiO-CaAlSiO), MgO-CaO (halite structure) binary, and on Al/Si ordered alkali feldspars (NaAlSiO-KAlSiO); as members for coupled substitutions, the diopside-jadeite pyroxenes (CaMgSiO-NaAlSiO) and diopside-CaTs pyroxenes (CaMgSiO-CaAlAlSiO) were chosen for testing and, as an application, the heat of mixing of the tremolite-glaucophane amphiboles (CaMgSiO(OH)-NaMgAlSiO(OH)) was computed. Six of these binaries were selected because of their experimentally well-known thermodynamic mixing behaviours. The comparison of the calculated heat of mixing data with calorimetric data showed good agreement for halite-sylvite, pyrope-grossular, and diopside-jadeite binaries and small differences for the Al/Si ordered alkali feldspar solid solution. In the case of the diopside-CaTs binary, the situation is more complex because CaTs is an endmember with disordered cation distributions. Good agreement with the experimental data could be, however, achieved assuming a reasonable disordered state. The calculated data for the Al/Si ordered alkali feldspars were applied to phase equilibrium calculations, i.e. calculating the Al/Si ordered alkali feldspar solvus. This solvus was then compared to the experimentally determined solvus finding good agreement. The solvus of the MgO-CaO binary was also constructed from DFT-based data and compared to the experimentally determined solvus, and the two were also in good agreement. Another application was the determination of the solvus in tremolite-glaucophane amphiboles (CaMgSiO(OH)-NaMgAlSiO(OH)). It was compared to solvi based on coexisting amphiboles found in eclogites and phase equilibrium experiments.
采用密度泛函理论（DFT）结合单缺陷方法进行计算，以确定矿物固溶体相的混合热行为。该方法的准确性在石盐 - 钾盐（NaCl - KCl）二元体系、镁铝榴石 - 钙铝榴石石榴石（MgAl₂SiO₄ - Ca₃Al₂SiO₄）、MgO - CaO（石盐结构）二元体系以及Al/Si有序碱性长石（NaAlSi₃O₈ - KAlSi₃O₈）上进行了测试；作为耦合替代的成员，选择透辉石 - 硬玉辉石（CaMgSi₂O₆ - NaAlSi₂O₆）和透辉石 - 钙钛辉石（CaMgSi₂O₆ - CaAl₂Al₂O₆）进行测试，并作为应用实例，计算了透闪石 - 蓝闪石角闪石（Ca₂Mg₅Si₈O₂₂(OH)₂ - Na₂Mg₃Al₂Si₈O₂₂(OH)₂）的混合热。选择其中六个二元体系是因为它们具有实验上已知的热力学混合行为。将计算得到的混合热数据与量热数据进行比较，结果表明石盐 - 钾盐、镁铝榴石 - 钙铝榴石和透辉石 - 硬玉辉石二元体系的数据吻合良好，而Al/Si有序碱性长石固溶体的数据存在较小差异。在透辉石 - 钙钛辉石二元体系中，情况更为复杂，因为钙钛辉石是一种阳离子分布无序的端员矿物。然而，假设合理的无序状态，可以与实验数据取得良好的吻合。将Al/Si有序碱性长石的计算数据应用于相平衡计算，即计算Al/Si有序碱性长石的固溶曲线。然后将该固溶曲线与实验测定的固溶曲线进行比较，并发现两者吻合良好。MgO - CaO二元体系的固溶曲线也根据基于DFT的数据构建，并与实验测定的固溶曲线进行比较，两者同样吻合良好。另一个应用是确定透闪石 - 蓝闪石角闪石（Ca₂Mg₅Si₈O₂₂(OH)₂ - Na₂Mg₃Al₂Si₈O₂₂(OH)₂）的固溶曲线。将其与基于榴辉岩中共存角闪石和相平衡实验得到的固溶曲线进行了比较。
