Abstract Accurate knowledge of rare earth elements (REE) speciation in high pressure – high temperature fluids is required to model REE transport and precipitation in subduction zones and magmatic-hydrothermal environments, and the formation of rare metal deposits. Recent experiments (lanthanum, ytterbium, erbium) have demonstrated that REE chloride complexes are the main REE form in many hydrothermal fluids (Migdisov et al., 2016). However, the speciation of yttrium (Y(III)), a cation with an ionic radius similar to that of Ho(III), remains poorly constrained in chloride-rich hydrothermal solutions. We used ab initio molecular dynamics (MD) simulations to calculate the nature of Y(III)-Cl complexes and the thermodynamic properties of these species at temperatures up to 500 °C and pressures of 800 bar and 1000 bar. The MD results were complemented by in situ X-ray absorption spectroscopy (XAS) measurements. Our results indicate that at temperatures below 200 °C, chloro-complexes do not form readily, even in highly concentrated brines. At ambient condition, the Y(III) aqua ion binds to eight water molecules in a square antiprism geometry, which is consistent with previous ab initio studies (Ikeda et al., 2005a). The thermodynamic integration method was employed to calculate the formation constants (log K Θ ) of Y(III)-Cl− complexes in two simulation boxes containing different Y:Cl ratios; we obtained very consistent results of the standard log K Θ of the individual complexes from the two independent calculations, which confirms that the thermodynamic integration method is reliable and not significantly affected by technical limitations in box size, box composition, or simulation time. Based on the derived formation constants, we fit modified Ryzhenko–Bryzgalin (MRB) equation of state parameters, which enable extrapolation of the formation constants at elevated temperature and pressure. The results are consistent with the XAS data, and show that the stability of Y(III)-Cl complexes increases with increasing temperature, Y(III) forming high order Cl− complexes (up to YCl4−) in high salinity solutions at high temperature and pH = 3. We also compare the extrapolated log K Θ with the available data for other REE at 150 °C, 200 °C and 250 °C. At 200 °C, yttrium behaves more like a heavy REE, but from 200 °C to 250 °C, the formation constants of Y(III)-Cl complexes increase dramatically and behave more like the light REE. The difference of Cl− dominant species between Ho(III) (HoCl2+) and Y(III) (YCl2+) may account for the formation of anomalous Y/Ho ratios in some hydrothermal environments.

中文翻译：

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富含氯化物的热液中的钇络合和水合：结合从头分子动力学和原位 X 射线吸收光谱研究

摘要 要模拟俯冲带和岩浆-热液环境中的稀土元素输运和沉淀，以及稀有金属矿床的形成，需要准确了解高压高温流体中稀土元素 (REE) 的形态。最近的实验（镧、镱、铒）表明，氯化稀土复合物是许多热液流体中的主要 REE 形式（Migdisov 等，2016）。然而，钇 (Y(III)) 是一种离子半径与 Ho(III) 相似的阳离子，在富含氯化物的热液溶液中仍然受到很大限制。我们使用从头算分子动力学 (MD) 模拟来计算 Y(III)-Cl 配合物的性质以及这些物质在高达 500 °C 的温度和 800 bar 和 1000 bar 的压力下的热力学性质。MD 结果得到了原位 X 射线吸收光谱 (XAS) 测量的补充。我们的结果表明，在低于 200 °C 的温度下，即使在高浓度盐水中，也不容易形成氯络合物。在环境条件下，Y(III) 水离子以方形反棱镜几何形状与八个水分子结合，这与之前的 ab initio 研究一致（Ikeda 等人，2005a）。采用热力学积分法计算Y(III)-Cl-配合物在两个不同Y:Cl比的模拟箱中的形成常数(log K Θ)；我们从两个独立的计算中获得了单个复合物的标准 log K Θ 的非常一致的结果，这证实了热力学积分方法是可靠的，并且不受框尺寸技术限制的显着影响，框组成，或模拟时间。基于导出的地层常数，我们拟合了修正的 Ryzhenko-Bryzgalin (MRB) 状态参数方程，这使得能够在升高的温度和压力下外推地层常数。结果与XAS数据一致，表明Y(III)-Cl配合物的稳定性随温度升高而增加，Y(III)在高温高盐溶液中形成高阶Cl-配合物（高达YCl4-）和 pH = 3。我们还将外推 log K Θ 与其他 REE 在 150 °C、200 °C 和 250 °C 下的可用数据进行比较。在 200°C 时，钇更像重稀土元素，但从 200°C 到 250°C，Y(III)-Cl 配合物的形成常数急剧增加，更像轻稀土元素。