Abstract

Rare Earth Elements (REE) are classified as critical minerals and materials essential 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 REE speciation, solubility, and transport in ore deposits and for extraction, chemical processing, and recycling. However, the standard Gibbs free energy of formation (∆Gof,REEX) for these solids vary by 10s kJ mol−1 across different sources from literature. We applied the Sverjensky linear free energy relationship (LFER) to evaluate internal consistency for isostructural REE solid groups and predicted ∆Gof values for solid endmembers for which no experimental measurements are available. By accounting for both ionic radius effects and corresponding aqueous ion properties, the LFER equation,ΔGf,REEXo−βREEX⋅rREEZ+=aREEXΔGn,REEZ+o+bREEX[jls-end-space/]enables accurate ∆Gof value estimates, mostly within 5 kJ mol−1. 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 anionic component of the compound REEX. In this study, the parameters aREEX, bREEX, and βREEX were regressed from ∆Gof compilations in the literature for 16 isostructural groups of REE solids. Based on these linear relationships, we recommend a set of internally consistent ∆Gof,REEX values for the REE3+ aqua ions from Shock et al. (1997), and the fitted 142 endmembers of REE oxides, hydroxides, chlorides, fluorides, carbonates, ferrites, and zirconates. These ∆Gof,REEX values were combined with experimental or predicted values of So, Vo, and Cpo from the literature and incorporated into a new dataset, enabling the prediction of thermodynamic properties at high temperature and pressure (e.g., up to 1000 °C and 5 kbar). The log Ksp of REE solid dissociation reactions are incorporated into a thermodynamic dataset compatible with the USGS program PHREEQC and the MINES thermodynamic database for use with the GEMS code package. These efforts aid in modeling speciation, solubility, and reactive transport in natural and engineered systems.

Department(s)

Electrical and Computer Engineering

Publication Status

Full Text Access

Keywords and Phrases

Gibbs free energy; Internal consistency; Linear free energy relationships; REE solids; Thermodynamic database

International Standard Serial Number (ISSN)

0009-2541

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2026 Elsevier, All rights reserved.

Publication Date

05 Aug 2026

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