DEVELOPMENT OF AN INTERNALLY CONSISTENT THERMODYNAMIC DATABASE FOR REE MINERALS AND CRYSTALLINE SOLIDS

Rare Earth Elements (REE) are critical minerals (metals) for the transition from fossil fuels to renewable and clean energy. Accurate thermodynamic properties of REE minerals and other crystalline solids are crucial for geochemical modeling of the solubility, speciation, and transport of REE in ore formation, extraction, chemical processing, and recycling processes. However, the Gibbs free energies of formation (∆G^o_{f, REEX}) for these solids from different sources vary by 10s kJ/mol. We applied the Sverjensky linear free energy relationship (LFER) ¹ to evaluate and predict the ∆G^o_f of REE solids. ^2,3

Based on the linear relationships, we recommend a set of internally consistent ∆G^o_{f, REEX} for 155 end-members of REE phosphates, oxides, hydroxides, chlorides, fluorides, carbonates, hydrous carbonates, and ferrites. These ∆G^o_{f, REEX} are combined with experimental or predicted values of S^o, V^o, and Cp^o from the literature and incorporated into a new SUPCRT database SUPCRTBL_REE, which allows the calculations of thermodynamic properties to high P-T conditions (e.g., 1000 ^oC and 5 kb) at https://models.earth.indiana.edu. The log Ks of REE mineral reactions were incorporated into a modified USGS PHREEQC program for calculation of speciation, solubility, and reactive transport up to 1000 ^oC and 5 kb. This work is part of a large collaboration on experimental and theoretical geochemistry to develop a comprehensive, internally consistent, and open-source thermodynamic database for REE minerals and aqueous species.

1 Sverjensky, D. A. & Molling, P. A. A linear free-energy relationship for crystalline solids and aqueous ions. Nature 356, 231-234 (1992). https://doi.org/10.1038/356231a0

2 Pan, R. G., Zhu, C. Linear correlations of Gibbs free energy for rare earth element oxide, hydroxide, chloride, fluoride, carbonate, and ferrite minerals and crystalline solids. Geochimica et Cosmochimica Acta submitted (2024). https://doi.org/10.48550/arXiv.2405.03515

3 Pan, R. G. et al. Linear Correlations of Gibbs Free Energy of REE Phosphates (Monazite, Xenotime, and Rhabdophane) and Internally Consistent Binary Mixing Properties. Minerals 14 (2024). https://doi.org/10.3390/min14030305