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

稀土元素(REE)对美国经济、可再生能源和国家安全的未来至关重要。增强环境可持续性、国防能力和消费产品的新技术大幅增加了对稀土的需求。然而，美国的稀土供应主要依赖于从国外进口。此外，许多国内矿藏在采矿过程中受到高浓度钍(Th)和铀(U)污染环境的影响。为了解决这个问题，有必要发现在没有Th或U的情况下，在自然界中形成经济集中的REE所需的条件；这需要了解Th和U如何与矿物中的REE混合，以及如何避免这种混合。该项目将通过实验和地质模拟相结合的方法，探索Th和U如何与矿物中的稀土元素混合。这些结果将使预测贫Th和U的稀土矿物形成的自然环境成为可能，从而为地质学家提供他们所需的信息，以制定战略来勘探和定位可经济和安全开采的稀土矿藏。该项目还旨在教育高中生、本科生和研究生，并为他们成为科学家做好准备。综合教育计划致力于为高中生举办夏季地球化学学校，通过参观国家实验室和虚拟讲座来促进对学生的地球化学教育，并让学生参与研究项目中使用的实验和建模方法。这项研究计划的目标是产生新的知识，使之能够识别贫化Th和U的稀土磷矿形成的条件。在美国，从矿石中回收稀土的主要障碍之一是在加工和精炼过程中产生的放射性，这是由于主要稀土矿石矿物独居石和辉石中存在高浓度的Th和U。因此，迫切需要寻找贫化Th、U的稀土矿石，这反过来又取决于对其形成条件(如温度、压力、氧逸度、pH等)的更好了解，特别是那些Th和U在磷酸盐结构中的结合最少的矿石。由于稀土成矿的热液条件，稀土、U和Th的水溶液物种的性质现在已相当为人所知，但几乎没有关于磷酸盐基稀土/U/Th固溶体的信息。一个主要的挑战是正确地解释由于Th和U与稀土元素混合而导致的热力学非理想性，这往往被错误地假设为理想的，因此可能导致不准确或错误的预测。该项目将建立准确的热力学模型，描述稀土磷酸盐中Th和U的结合，这是地球化学建模人员预测热液系统中REE/U/Th的动员、分馏和沉积所必需的。所获得的知识将有助于开发新的勘探技术，从而能够识别和定位贫化Th和U的稀土矿。该项目还为学生提供了独特的机会，让他们接受跨学科的教育和培训，成为具有广泛思维和技能的地球化学家。这包括为高中生和教师开设的夏季地球化学学校，以培养他们对稀土元素和其他关键金属的地球化学的兴趣。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.