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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 (Ce1−xREExPO4·nH2O; 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 (ΔHex) 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 ΔHex values with a slight asymmetry; (2) binary Ce-Nd and Ce-Sm which display negative ΔHex values with a nearly symmetric shape; (3) Ce-Eu and Ce-Gd which display both negative and positive ΔHex values with nearly symmetric shape. The excess Gibbs energy (ΔGex) 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 ΔGex values combined with the calorimetric ΔHex 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 Ce3+. These observations corroborate with the trends in the Raman v1 stretching bands of the PO4-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. 

Rare earth elements (REE) are critical elements found in monazite, xenotime, and hydrated REE phosphates which typically form in hydrothermal mineral deposits. Accurate predictions of the solubility of these REE phosphates and the speciation of REE in aqueous fluids are both key to understanding the controls on the transport, fractionation, and deposition of REE in natural systems. Previous monazite and xenotime solubility experiments indicate the presence of large discrepancies between experimentally derived solubility constants versus calculated solubilities by combining different data sources for the thermodynamic properties of minerals and aqueous species at hydrothermal conditions. In this study, these discrepancies were resolved by using the program GEMSFITS to optimize the standard partial molal Gibbs energy of formation (ΔfG°298) of REE aqueous species (REE3+ and REE hydroxyl complexes) at 298.15 K and 1 bar while keeping the thermodynamic properties fixed for the REE phosphates. A comprehensive experimental database was compiled using solubility data available between 25 and 300 °C. The latter permits conducting thermodynamic parameter optimization of ΔfG°298 for REE aqueous species. Optimal matching of the rhabdophane solubility data between 25 and 100 °C requires modifying the ΔfG°298 values of REE3+ by 1–6 kJ/mol, whereas matching of the monazite solubility data between 100 and 300 °C requires modifying the ΔfG°298 values of both REE3+ and REEOH2+ by ∼ 2–10 kJ/mol and ∼ 15–31 kJ/mol, respectively. For xenotime, adjustments of ΔfG°298 values by 1–26 kJ/mol are only necessary for the REE3+ species. The optimizations indicate that the solubility of monazite in acidic solutions is controlled by the light (L)REE3+ species at <150 °C and the LREEOH2+ species at >150 °C, whereas the solubility of xenotime is controlled by the heavy (H)REE3+ species between 25 and 300 °C. Based on the optimization results, we conclude that the revised Helgeson-Kirkham-Flowers equation of state does not reliably predict the thermodynamic properties of REE3+, REEOH2+, and likely other REE hydroxyl species at hydrothermal conditions. We therefore provide an experimental database (ThermoExp_REE) as a basic framework for future updates, extensions with other ligands, and optimizations as new experimental REE data become available. The optimized thermodynamic properties of aqueous species and minerals are available open access to accurately predict the solubility of REE phosphates in fluid-rock systems. 

The rare earth elements (REE) are essential for the high-tech and green technology industries, and used, for example, in computers, smartphones, and wind turbines. The REE are considered critical minerals and can be highly enriched in certain magmatic-hydrothermal systems including alkaline complexes and carbonatites. Almost all of the critical mineral deposits show a complex overprint by hydrothermal processes during their genesis. However, our understanding of the mobility in these ore- forming systems and our knowledge about the stability of REE minerals is still very limited. The MINES thermodynamic database is an open-access database and continuously updated with the most up to date thermodynamic data for REE aqueous species and minerals. This database also includes rock-forming minerals and permits simulating the mineralogy and alteration geochemistry that relates to the formation of these critical mineral deposits. This study gives a short overview of the MINES thermodynamic database and the GEMS code package for simulating the formation of hydrothermal calcite, fluorite and bastnäsite-(Ce) veins relevant to interpreting critical mineral deposits. 

The MINES thermodynamic database (version 23) is a revised internally consistent thermodynamic dataset for minerals, aqueous species, and gases for simulating geochemical processes at hydrothermal conditions (≤5 kbar and ≤600 °C) with a focus on ore forming processes. The database follows a rolling release approach with new file versions becoming available once updated. The version number corresponds to the year of the most recent file creation and the number after the decimal separator indicates an upgrade during the year of release. The database is currently intended to be used with the GEMS geochemical modeling program ( http://gems.web.psi.ch/ ). Future versions will include human-readable data in .xlsx, .csv, and JSON files with all the data values, units, and references. 

Monazite is a light rare earth element (REE) phosphate found in REE mineral deposits, such as those formed in (per)alkaline and carbonatite magmatic-hydrothermal systems, where it occurs in association to the development of alteration zones and hydrothermal veins. Although it has been recognized that monazite may undergo replacement by coupled dissolution-precipitation processes, currently there is no model describing the compositional REE variations in monazite resulting from direct interaction with or precipitation from hydrothermal fluids. To develop such a model requires quantification of the thermodynamic properties of the aqueous REE species and the properties of the monazite endmembers and their solid solutions. The thermodynamic properties of monazite endmembers have been determined previously using calorimetric methods and low temperature solubility studies, but only a few solubility studies have been conducted at >100 °C. In this study, the solubility products (logKs0) of LaPO4, PrPO4, NdPO4, and EuPO4 monazite endmembers have been measured at temperatures between 100 and 250 °C and saturated water vapor pressure. The solubility products are reported with an uncertainty of ±0.2 (95% confidence) according to the reaction, REEPO4(s) = REE3+ + PO43−. (see table in manuscript) The REE phosphates display a retrograde solubility, with the measured Ks0 values varying several orders of magnitude over the experimental temperature range. Discrepancies were observed between the experimental solubility products and the calculated values resulting from combining calorimetric data of monazite with the properties of the aqueous REE3+ and PO43− species available in the literature. The differences between the calculated and measured standard Gibbs energy of reaction (ΔrG0) for PrPO4, NdPO4, and EuPO4 increased with higher temperatures (up to 15 kJ mol−1 at 250 °C), whereas for LaPO4 these differences increased at lower temperatures (up to 8 kJ mol−1 at 100 °C). To reconcile these discrepancies, the standard enthalpy of formation (ΔfH0) of monazite was optimized by fitting the experimental solubility data and extrapolating these fits to reference conditions of 25 °C and 1 bar. The optimized thermodynamic data provide the first internally consistent dataset for the solubility of all the monazite endmembers, and can be used to model REE partitioning between monazite and hydrothermal fluids at >100 °C. 

Monazite-(Ce) and xenotime-(Y) occur as secondary minerals in iron-oxide-apatite (IOA) deposits, and their stability and composition are important indicators of timing and conditions of metasomatism. Both of these minerals occur as replacement of apatite and display slight but important variations in light (e.g. La, Ce, Pr, Nd, etc.) and heavy (e.g. Y, Er, Dy, Yb, etc.) REE concentrations [1,2]. The causes for these chemical variations can be quantified by combining thermodynamic modeling with field observations. Major challenges for determining the stability of these minerals in hydrothermal solutions are the underlying models for calculating the thermodynamic properties of REE-bearing mineral solid solutions and aqueous species as a function of temperature and pressure. The thermodynamic properties of monazite and xenotime have been determined using several calorimetric methods [3], but only a few hydrothermal solubility studies have been undertaken, which test the reliability and compatibility of both the calorimetric data and thermodynamic properties of associated REE aqueous species [4,5]. Here, we evaluate the conditions of REE metasomatism in the Pea Ridge IOA-REE deposit in Missouri, and combine newly available experimental solubility data to simulate the speciation of LREE vs. HREE, and the partitioning of REE as a function of varying fluid compositions and temperatures. Our new experimental data will be implemented in the MINES thermodynamic database (http:// tdb.mines.edu) for modeling the chemistry of crustal fluid-rock equilibria [6]. [1] Harlov et al. (2016), Econ. Geol. 111, 1963-1984;[2] Hofstra et al. (2016), Econ. Geol. 111, 1985-2016; [3] Navrotsky et al. (2015), J. Chem. Thermodyn. 88, 126-141; [4] Gysi et al. (2015), Chem. Geol. 83-95; [5] Gysi et al. (2018), Geochim. Cosmochim. Acta 242, 143-164; [6] Gysi (2017), Pure and Appl. Chem. 89, 581-596. 

Monazite (CePO4) is a light rare earth element (REE) phosphate occurring as accessory mineral in metamorphic, igneous and sedimentary rocks, and is also a common mineral in REE mineral deposits. Metasomatism of monazite yields important clues about fluid-rock interaction in the crust, in particular, because its compositional variations may enable us to determine conditions of mineralization. The thermodynamic properties of monazite have been determined using several calorimetric methods, but up to the present time only a few solubility studies have been undertaken, which test the reliability of both, the thermodynamic properties of the REE phosphates and associated REE aqueous species. In this study, we have measured the solubility of the monoclinic REE phosphate end-members CePO4, SmPO4, and GdPO4 in aqueous perchloric acid solutions at temperatures from 100 to 250 °C at saturated water vapor pressure (swvp). The solubility products (Ks0) were determined according to the reaction: REEPO4 = REE3+ + PO43−. Combining available calorimetric data for the REE phosphates with the REE aqueous species from the Supcrt92 (slop98.dat) dataset, yields several orders of magnitude differences when compared with our solubility measurements. We have investigated ways to reconcile these discrepancies and propose a consistent set of provisional thermodynamic properties for REE aqueous species and REE phosphates that reproduce our measured solubility values. To reconcile these discrepancies, we have used the GEMS code package and GEMSFITS for parameter optimization by adjusting the standard Gibbs energy of REE3+ and REEOH2+ at 25 °C and 1 bar. An alternative optimization could involve adjustment of the standard Gibbs energy of REEPO4(s) and REEOH2+. Independently of the optimization method used, this study points to a need to revise the thermodynamic properties of REEOH2+ and possibly other REE hydroxyl species in future potentiometric studies. These revisions will have an impact on calculated solubilities of REE phosphates and our understanding of the mobility of REE in natural hydrothermal fluids. 

CePO4 and YPO4 are major components in monazite and xenotime, respectively, which are common hydrothermal phases in REE mineral deposits. Both minerals also occur as secondary minerals in iron-oxide-apatite deposits [1,2], and as accessory phases in high-grade metamorphic rocks where they display varying degrees of metasomatism. Studying the cause of their compositional variations using thermodynamic modeling may provide geochemical signals for interpreting P- T-x of crustal fluid-rock interaction. The thermodynamic properties of monazite and xenotime have been determined using several calorimetric methods [3], but only a few solubility studies have been undertaken, which test the reliability of both the calorimetric data and thermodynamic properties of associated REE aqueous species [4]. Combining available calorimetric data with the REE aqueous species from Haas et al. [5], implemented in the Supcrt92 database [6], yields several orders of magnitude differences when compared with our solubility measurements. To reconcile these discrepancies, we have used the GEMS code package [7,8] and GEMSFITS [9] for parameter optimization, and re- evaluated the standard Gibbs energies for aqueous REE species, while maintaining consistency with available calorimetric measurement of the REE phosphates. This study points to a need to revise the thermodynamic properties of the REE hydroxyl species, which will have an impact on the calculated solubilities of the REE phosphates and our understanding of the mobility of REE in hydrothermal fluids. Our new experimental data will be implemented in the MINES thermodynamic database (http:// tdb.mines.edu) [10] for modeling the chemistry of crustal fluid-rock equilibria.