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。Xen-time-(Y)和独居石-(Ce)是地壳中含有稀土元素(REE)的常见矿物。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的时间和温度的重要地质示踪剂。从不同地质背景下的稀土磷酸盐中提取信息的全部潜力才刚刚开始探索，稀土矿物的性质在寻找关键金属方面获得了越来越大的经济兴趣，这些金属在新兴的高科技和绿色产业中发挥着重要作用。这项研究将实验室实验和数值模拟相结合，建立了一种新的热力学流体-矿物固溶体模型，可用于地质背景下的矿床和变质/交代过程的研究。这项研究将通过在可开放获取的采矿热力学数据库(http://tdb.mines.edu/)，)中执行所产生的数据来传播，并将在专用网页上提供一系列教育建模项目。将在科罗拉多矿业学院(CSM)组织一个关于稀土矿藏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调查员新的地壳流体-岩石实验室来促进一名早期职业科学家，他将在那里培训2名研究生，并让CSM和/或巴西科学流动方案的暑期本科生参与进来。利用数值模拟，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。由于稀土矿物固溶体热力学数据的缺乏，我们目前对稀土在水中流体和矿物中的行为的理解是有限的。拟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热、端元的溶解度和它们的热容。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能量最小化计划GEMS中实施，并应用于研究热液稀土矿藏的成因。该项目将试图跨越变质岩石学和地球化学之间的界限，并有望对解释稀土在地壳流体中的行为产生影响。这一知识可扩展到确定主要配体对地壳中金属活动性的影响，并可应用于辅助矿物固溶体的组成可用于追踪变质过程中的交代阶段的系统。

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.