Rhabdophane is a hydrous phosphate that commonly replaces monazite as a weathering product in critical mineral deposits during the alteration of rare earth elements (REE) bearing carbonatites and alkaline igneous complexes. It is an important host to the light (L)REE (i.e., La to Gd) but the stability and structure of binary solid solutions between the Ce and the other LREE endmembers have not yet been determined experimentally. Here we present room temperature calorimetric experiments that were used to measure the enthalpy of precipitation of rhabdophane (CeREEPO·HO; REE = La, Pr, Nd, Sm, Eu, and Gd). The solids were characterized using X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and the role of water in the rhabdophane structure was further determined using thermogravimetric analysis coupled with differential scanning calorimetry. The calorimetric experiments indicate a non-ideal behavior for all of the binary solid solutions investigated with an excess enthalpy of mixing (Δ) described by a 2- to 3-term Guggenheim parameters equation. The solid solutions were categorized into three groups: (1) binary Ce-La and Ce-Pr which display positive Δ values with a slight asymmetry; (2) binary Ce-Nd and Ce-Sm which display negative Δ values with a nearly symmetric shape; (3) Ce-Eu and Ce-Gd which display both negative and positive Δ values with nearly symmetric shape. The excess Gibbs energy (Δ) of the solid solutions was further investigated using a thermodynamic analysis approach of aqueous-solid solution equilibria and the optimization programs GEMS and GEMSFITS. The resulting Δ values combined with the calorimetric Δ values indicate that there is likely an excess entropy contribution implying important short-range structural modifications in the solid solutions dependent on the deviation of the REE ionic radii from the size of Ce. These observations corroborate with the trends in the Raman stretching bands of the PO-site. The excess molar volumes determined from X-ray diffraction analysis further indicate an overall asymmetric behavior in all of the studied binary solid solutions, which becomes increasingly important from La to Gd. The pronounced short-range order–disorder occurring in groups 2 and 3 solid solutions mimics some of the behavior observed from previous studies in anhydrous monazite solid solutions. This study highlights the potential to use the chemistry and the structural modifications of rhabdophane as potential indicators of formation conditions in geologic systems and permits improving our modeling capabilities of REE partitioning in critical minerals systems.

中文翻译：

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非理想二元弹多菲烷固溶体(Ce1−xREExPO4·nH2O)的反应量热和结构晶体性质

弹状多芬是一种水合磷酸盐，在含有稀土元素 (REE) 的碳酸岩和碱性火成岩复合体发生蚀变过程中，通常取代独居石作为关键矿床中的风化产物。它是轻 (L)REE（即 La 到 Gd）的重要宿主，但 Ce 和其他轻稀土端元之间的二元固溶体的稳定性和结构尚未通过实验确定。在这里，我们介绍了用于测量弹状多芬（CeREEPO·H2O；REE = La、Pr、Nd、Sm、Eu 和 Gd）沉淀焓的室温量热实验。使用 X 射线衍射、扫描电子显微镜、拉曼光谱对固体进行了表征，并使用热重分析和差示扫描量热法进一步确定了水在弹状蛋白烷结构中的作用。量热实验表明，使用 2 至 3 项古根海姆参数方程描述的过量混合焓 (Δ) 研究的所有二元固溶体均表现出非理想行为。固溶体分为三组：（1）二元Ce-La和Ce-Pr，显示出正Δ值，但略有不对称性； (2)二元Ce-Nd和Ce-Sm，其显示负Δ值且形状接近对称； （3）Ce-Eu和Ce-Gd显示负和正Δ值且形状接近对称。使用水-固溶体平衡的热力学分析方法以及优化程序 GEMS 和 GEMSFITS 进一步研究了固溶体的过量吉布斯能 (Δ)。所得的 Δ 值与量热 Δ 值相结合表明，可能存在过量的熵贡献，这意味着固溶体中存在重要的短程结构修改，具体取决于 REE 离子半径与 Ce 尺寸的偏差。这些观察结果证实了 PO 位点拉曼伸缩带的趋势。 X 射线衍射分析确定的过量摩尔体积进一步表明所有研究的二元固溶体中的整体不对称行为，从 La 到 Gd，这一点变得越来越重要。第 2 组和第 3 组固溶体中发生的明显短程有序无序模仿了之前在无水独居石固溶体研究中观察到的一些行为。这项研究强调了利用弹状多芬的化学和结构修饰作为地质系统中形成条件的潜在指标的潜力，并允许提高我们对关键矿物系统中稀土元素分配的建模能力。