Developing the thermodynamic solid solution models for Th, U, REE phosphates needed to identify the formation conditions of Th, U-depleted REE ores

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Rare earth elements (REE) are critical for the future of the U.S. economy, renewable energy, and national security. New technologies enhancing environmental sustainability, defense capability, and consumer products have sharply increased the demand for the REE. However, the supply of REE in the U.S. relies mostly on their import from foreign sources. Furthermore, many of the domestic deposits suffer from high concentrations of thorium (Th) and uranium (U) that contaminate the environment during mining. To deal with this issue, it is necessary to discover the conditions needed for economic concentrations of the REE to form in nature in the absence of Th or U; this requires understanding how Th and U mix with the REE in minerals and how such mixing can be avoided. This project will explore how Th and U mix with the REE in minerals through a combination of experiments and geological modeling. These results will make it possible to predict the natural environments in which Th- and U-poor REE minerals form and thereby provide geologists with the information they need to develop strategies to explore for and locate deposits of REE that can be economically and safely mined. This project also aims to educate high school, undergraduate, and graduate students in geochemistry, and prepare them for careers as scientists. The integrated education plan is committed to holding summer geochemistry schools for high school students, promoting geochemical education to student through visits to national laboratories and virtual lectures, and engaging students in the experimental and modeling methods that are used in the research project.The objective of this research proposal is to generate new knowledge enabling identification of the conditions under which Th and U-depleted REE phosphate ores can form. One of the major impediments to the recovery of REE from ores in the U.S. is the radioactivity generated during their processing and refining due to the presence of high concentrations of Th and U in the main REE ore minerals, monazite and xenotime. Thus, there is a strong need to find Th, U-depleted REE ores, which, in turn, depends on developing a better understanding of their formation conditions (e.g., temperature, pressure, oxygen fugacity, pH, etc.), particularly those for which the incorporation of Th and U into phosphate structures is minimal. Whereas the properties of the aqueous species of the REE, U, and Th are now reasonably well-known for the hydrothermal conditions of REE ore formation, there is almost no information on phosphate-based REE/U/Th solid solutions. A major challenge is to correctly account for thermodynamic non-ideality due to the mixing of Th and U with the REE, which is often incorrectly assumed to be ideal and thus can lead to inaccurate or false predictions. This project will establish accurate thermodynamic models describing the incorporation of Th and U in REE phosphates that are critically needed by geochemical modelers to predict the mobilization, fractionation, and deposition of REE/U/Th in hydrothermal systems. The knowledge obtained will enable the development of new exploration techniques permitting the identification and localization of Th- and U-depleted REE ores. The project also offers unique opportunities for students to receive the interdisciplinary education and training needed to become geochemists with broad mindsets and skillsets. This includes summer geochemistry schools for high school students and teachers to develop their interest in the geochemistry of the REE and other critical metals.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.