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 (∆Gof, REEX) for these solids from different sources vary by 10s kJ/mol. We applied the Sverjensky linear free energy relationship (LFER) to evaluate their internal consistency and predict the unavailable ∆Gof of the REE solids. By considering both the effects of ionic radius size and corresponding aqueous ion properties, the Sverjensky LFER,
allows estimates with much accuracy and precision. Here, rREEZ+ represents the Shannon-Prewitt ionic radii (Å) of REEZ+, and ∆Gon, REEZ+ denotes the non-solvation contribution to the ∆Gof of the aqueous REEZ+ ion. X represents the remainder of the compounds. In this study, the parameters aREEX, bREEX, and βREEX were regressed from ∆Gof compilations in the literature for 13 isostructural families. Based on these linear relationships, we recommend a set of internally consistent ∆Gof, REEX for 119 end-members of REE oxides, hydroxides, chlorides, fluorides, carbonates, hydrous carbonates, and ferrites. These ∆Gof, REEX are combined with experimental or predicted values of So, Vo, and Cpo from the literature and incorporated into a new SUPCRT database, which allows the calculations of thermodynamic properties to high P-T conditions (e.g., up to 1000 oC and 5 kb). The log Ksp of REE solid dissociation reactions were incorporated into a modified USGS program PHREEQC for calculations of speciation, solubility, and reactive transport. These thermodynamic databases will also be incorporated into the MINES database to be used together with the GEMS code package in the future.
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Title:
Linear correlations of Gibbs free energy for rare earth element oxide, hydroxide, chloride, fluoride, carbonate, and ferrite minerals and crystalline solids