Optimized thermodynamic properties of REE aqueous species (REE³⁺and REEOH²⁺) and experimental database for modeling the solubility of REE phosphate minerals (monazite, xenotime, and rhabdophane) from 25 to 300C

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 (Delta(f)G degrees(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 degrees C. The latter permits conducting thermodynamic parameter optimization of Delta(f)G degrees(298) for REE aqueous species. Optimal matching of the rhabdophane solubility data between 25 and 100 degrees C requires modifying the Delta(f)G degrees(298) values of REE3+ by 1-6 kJ/mol, whereas matching of the monazite solubility data between 100 and 300 degrees C requires modifying the Delta(f)G degrees(298) values of both REE3+ and REEOH2+ by similar to 2-10 kJ/mol and similar to 15-31 kJ/mol, respectively. For xenotime, adjustments of Delta(f)G degrees(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 degrees C and the LREEOH2+ species at >150 degrees C, whereas the solubility of xenotime is controlled by the heavy (H)REE3+ species between 25 and 300 degrees 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.