The solubility of ErPO4 and Er speciation in hydrothermal fluids at varying pH and salinity between 350 and 450 °C

Kershaw, Charles T.; Hurtig, Nicole C.; Gysi, Alexander P.; Migdisov, Artas A.; Waters, Laura E.; Harlov, Daniel

doi:10.1016/j.gca.2026.02.010

The solubility of ErPO₄ and Er speciation in hydrothermal fluids at varying pH and salinity between 350 and 450 °C

Journal Article · Fri Feb 13 00:00:00 EST 2026 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2026.02.010· OSTI ID:3012126

^[1]; ^[1]; ^[1]; Migdisov, Artas A. ^[2]; Waters, Laura E. ^[1]; ^[3]

New Mexico Tech, Socorrow, NM (United States)
Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
Deutsches GeoForschungsZentrum GFZ, Potsdam (Germany); China Univ. of Geosciences, Wuhan (China); Univ. of Johannesburg (South Africa)

The rare earth elements (REE) are important for the green-energy transition and can be incorporated into the REE phosphates, such as xenotime-(Y), which also hosts heavy REE (Tb– Lu). Xenotime-(Y) is a common accessory mineral in metamorphic rocks and a range of mineral deposits where it controls the mobility of heavy REE, however, the impact of high temperature aqueous fluids on the behavior of heavy REE is largely unknown. Thermodynamic modeling can be utilized as a tool to predict the mobility of REE in hydrothermal aqueous fluids, but must be supported by accurate experimental data. Here, we measured the solubility of endmember synthetic xenotime-structured ErPO4 in NaCl-HCl-NaOH-bearing aqueous solutions at 350 °C and water vapor saturation pressure, at 400 and at 450 °C and 500 bar using batch-type Inconel reactors. Erbium speciation was investigated as a function of pH from 2.8 to 8, where Er chloride species are predominant at acidic conditions (pH <3) and Er hydroxyl complexes are predominant at near- neutral to alkaline conditions (pH >3). At pH 7–9, the measured ErPO₄ solubility (-9.8 to -7.5 log m_Er) is up to 2.5 orders of magnitude lower than thermodynamic predictions (-9.4 to -6.7 log m_Er) using existing thermodynamic databases. At pH 2–3, the predicted ErPO₄ solubility is ~0.5 orders of magnitude higher at 350 °C and ~1 order of magnitude lower at 450 °C compared to experimentally measured Er concentrations. The thermodynamic properties of aqueous Er species were therefore revised in this study. The partial molal Gibbs energy of formation (Δ_fG⁰_T,P) for aqueous Er hydroxyl and chloride species are optimized using GEMSFITS and the logarithmic formation constants (logβ_n(^Cl,OH)) were derived at each experimental temperature and pressure. The updated thermodynamic properties for Er hydroxyl species (Er(OH)⁺², Er(OH)₂⁺, and Er(OH)₃⁰) show that their stability shifts to more acidic conditions at and below 400 °C. The Er chloride species (ErCl⁺² and ErCl₂⁺) show increased stability compared to Er hydroxyl species at temperatures of 450 °C and 0.01 mol/kg NaCl. The updated thermodynamic properties are implemented into the GEM-Selektor modeling package to investigate the mobility of Er in saline hydrothermal fluids in equilibrium with alkaline rocks. Importantly, the updated properties for Er hydroxyl species result in low Er solubility at rock equilibrated pH conditions due to an expanded hydroxyl predominance zone, but lower aqueous complex stability overall, whereas previous models suggest greater stability for aqueous Er species. Furthermore, ErPO₄ solubility increases with decreasing temperature due to the deprotonation of HCl, which increases the acidity of hydrothermal fluids and the availability of Cl^- to complex with the REE. These simulations highlight how fluid-rock reaction and temperature affect the mobility of REE in hydrothermal ore-forming systems.

View Accepted Manuscript (DOE)

Research Organization:: New Mexico Institute of Mining and Technology, Socorro, NM (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Basic Energy Sciences (BES)

Grant/Contract Number:: SC0022269

Other Award/Contract Number:: NSF/EAR-MRI 2117061

OSTI ID:: 3012126

Journal Information:: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Vol. 419; ISSN 0016-7037

Publisher:: Elsevier; The Geochemical Society; The Meteoritical SocietyCopyright Statement

Country of Publication:: United States

Language:: English

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The solubility of ErPO4 and Er speciation in hydrothermal fluids at varying pH and salinity between 350 and 450 °C

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The solubility of ErPO₄ and Er speciation in hydrothermal fluids at varying pH and salinity between 350 and 450 °C