Mobility of rare earth elements in fluid-mineral interactions: europium and cerium speciation in hydrothermally synthesized sulfate and sulfide minerals

Understanding the incorporation of rare earth elements (REE) into their principal host minerals is fundamental to tracing geological processes, improving ore deposit formation models, assessing the resource potential of a deposit, and designing effective and environmentally sustainable extraction processes. This study investigates the speciation and fractionation of europium (Eu) and cerium (Ce) in hydrothermally synthesized sulfate (anhydrite) and sulfide (sphalerite) minerals. Electron Probe Microanalysis reveals that Ce and Eu preferentially partition into anhydrite rather than sphalerite, consistent with the Goldschmidt rule. Furthermore, we show that Eu concentration and oxidation state are decoupled within single anhydrite crystals: synchrotron X-ray Near-edge structure (XANES) and microprobe cathodoluminescence (CL) data both show that Eu 3+ predominates in the core and Eu 2+ increases from the core to the rim, whereas Ce 3+ is the only oxidation state of Ce in anhydrite under the same conditions. In contrast, Eu and/or Ce concentrations oscillate from the core to the rim of anhydrite crystals, with the lowest concentration around the rim. Correlations among elemental concentrations in anhydrite show that Eu 3+ and Ce 3+ replace Ca 2+ via a coupled substitution with Na + , and Eu 2+ replaces Ca 2+ via an isomorphous substitution. Complementary ab initio molecular simulations indicate that the incorporation of Eu and Ce caused only small local distortions of the anhydrite crystal structure, with more substantial distortion commonly observed around vacancies and Na sites; the extent of distortion is proportional to the differences between the effective ionic radii of Ca 2+ and substituted Eu, Ce, and Na ions. This study demonstrates that hydrothermal anhydrite can incorporate up to several weight percent of REE, with a complex incorporation process controlled by the evolution of fluid composition and local fluid-mineral interactions. Spatially resolved oxidation state and concentration analyses provide deeper insights into fluid evolution than bulk REE measurements