The Thermodynamic Properties of Rare Earth Element (REE) Minerals: A Solid Solution Model for Xenotime-(Y) and Monazite-(Ce)

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The Rare-Earth Elements (REE) are among the most critical elements used in a variety of new technological applications. The minerals xenotime-(Y) and monazite-(Ce) are common minerals that contain these rare earth elements (REE) in the Earth's crust. In addition to their societal relevance, REE bearing minerals (phosphates) have proven to be important geological tracers for constraining timing and temperatures of crustal processes. The full potential for retrieving information from REE phosphates in different geological settings has just begun to be explored, and the properties of REE minerals have gained an increased economic interest in the search for critical metals, which play this major role in emerging high technology and green industries. The proposed study combines laboratory experiments with numerical modeling for building a new thermodynamic fluid-mineral solid solution model that can be applied to the study of ore deposits and metamorphic/metasomatic processes in geological settings. This research will be disseminated by implementing the resulting data in the open access MINES thermodynamic database (http://tdb.mines.edu/), and a set of educational modeling projects will be provided on a dedicated webpage. A short course on REE mineral deposits and numerical modeling will be organized at the Colorado School of Mines (CSM) to introduce the application of this database. This project will also promote an early career scientist by supporting the investigator's new crustal fluid-rock laboratory, where he will train 2 graduate students and involve summer undergraduate students from CSM and/or the Brazil Scientific Mobility program. Using numerical simulations, the team will predict the stability of minerals and fluids in the Earth's crust as a function of pressure and temperature, and expand these capabilities based on experiments and new theoretical models. Our current understanding of the behavior of REE in aqueous fluids and minerals is limited by the paucity of available thermodynamic data for REE mineral solid solutions. The proposed work will combine laboratory mineral solubility and calorimetric experiments to determine the enthalpy of mixing of binary REE phosphate solid solutions, the solubility of end members and their heat capacities. This will permit building an internally consistent thermodynamic dataset that will be implemented in the Gibbs energy minimization program GEMS, and applied to study the genesis of hydrothermal REE mineral deposits. The project will attempt to cross boundaries between metamorphic petrology and geochemistry, and is expected to have an impact for the interpretation of the behavior of REE in crustal fluids. This knowledge can be extended to determine the effects of major ligands on the mobility of metals in the crust, and may be applied to systems were the composition of accessory mineral solid solutions could be used to track metasomatic stages during metamorphism.