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

稀土元素（REE）是各种新技术应用中最关键的元素之一。xenotime-(Y)和monazite-(Ce)是地壳中常见的含有稀土元素的矿物。除了具有社会意义外，含稀土矿物（磷酸盐）已被证明是限制地壳过程时间和温度的重要地质示踪剂。在不同的地质环境中从稀土磷酸盐中获取信息的全部潜力才刚刚开始得到探索，在寻找关键金属方面，稀土矿物的特性获得了越来越大的经济利益，这些金属在新兴的高技术和绿色工业中起着重要作用。本研究将实验室实验与数值模拟相结合，建立了一种新的流体-矿物固溶体热力学模型，该模型可应用于矿床和地质环境中的变质/交代过程的研究。这项研究将通过在开放获取的地雷热力学数据库（http://tdb.mines.edu/）中执行所得数据来传播，并将在一个专门的网页上提供一套教育模型项目。科罗拉多矿业学院将举办一个关于稀土矿床和数值模拟的短期课程，介绍该数据库的应用。该项目还将通过支持研究者的新地壳流体-岩石实验室来培养一名早期职业科学家，他将在那里培养2名研究生，并包括来自CSM和/或巴西科学流动计划的夏季本科生。通过数值模拟，该团队将预测地壳中矿物和流体的稳定性作为压力和温度的函数，并在实验和新的理论模型的基础上扩展这些能力。我们目前对稀土在水流体和矿物中的行为的理解受到稀土矿物固体溶液可用热力学数据的缺乏的限制。建议的工作将结合实验室矿物溶解度和量热实验来确定二元稀土磷酸盐固溶体的混合焓，端元的溶解度和它们的热容量。这将允许建立一个内部一致的热力学数据集，该数据集将在Gibbs能量最小化程序GEMS中实施，并应用于热液REE矿床的成因研究。该项目将尝试跨越变质岩石学和地球化学之间的界限，并有望对地壳流体中稀土元素的行为解释产生影响。这种知识可以扩展到确定主要配体对地壳中金属迁移率的影响，并且可以应用于辅助矿物固溶体的组成可以用来跟踪变质作用期间的交代阶段的系统。

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.