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A major goal of this conference is to tackle the challenges described above and build a new network of scientists and professionals with different expertise, including but not limited to experimental geochemistry/chemistry, thermodynamic/geochemical modeling and databases, reactive mass transport modeling, molecular dynamic simulations, element extraction/separation technologies, theoretical thermodynamics and equations of state, and mineralogy, ore deposits, and processes in natural systems. Another important aspect is the participation of students and training the next generation of leaders in the field of critical minerals and thermodynamics. Participants at this meeting include scientists from academia and national laboratories, graduate and undergraduate students, as well as liaisons from industry, governmental agencies, and geological surveys. This five-day meeting includes daily talks, keynotes, small workshops, discussion sessions, and two evenings of poster sessions for students. One day includes an excursion to the nearby Lemitar Mountains carbonatite rare earth elements deposit to discover the geology of New Mexico and allow participants to link geosciences with other areas of basic energy sciences. We will also organize a geochemical modeling workshop using our “in-house” MINES thermodynamic database (Gysi et al., 2023) to show an application of thermodynamics to modeling critical mineral deposits. 

Critical mineral deposits commonly form in magmatic-hydrothermal systems including carbonatites and/or alkaline syenites, and more evolved peralkaline granites where the rare earth element (REE) undergo a complex array of partitioning, transport and mineralization. Significant alteration and veining zones develop in these deposits and can be used to vector ore zones in the field [1]. The REE ore minerals typically reflect the characteristics of these systems, which are enriched in carbonate, fluoride, and phosphate or a combination thereof. The REE can also be incorporated into vein minerals such as calcite, fluorite and apatite where the REE3+ exchange for Ca2+ on the crystal lattice [2]. These minerals give us clues about the hydrothermal reaction paths of REE in critical mineral deposits. This study aims to: 1) present our recent findings from hydrothermal fluid-mineral REE partitioning experiments, 2) discuss thermodynamic models to simulate REE in critical mineral deposits, and 3) link the thermodynamic simulations to field observations. Hydrothermal fluid-calcite partitioning experiments were conducted between 100 and 200 °C by hydrothermal fluid mixing and precipitation [2] at near neutral to mildly alkaline pH (6 – 9). The REE concentrations in synthetic calcite crystals and aqueous fluids sampled in situ were used to fit the data to the lattice strain model [3] and using the Dual Thermodynamic approach [4]. A second type of experiment consisted of reacting natural fluorite and apatite crystals with acidic to mildly acidic (pH of 2 – 4) aqueous fluids in batch-type reactors to study the behavior of REE and mineral dissolution-precipitation reactions near crystal surfaces. The GEMS code package [5] was used to implement these new data into a thermodynamic model and simulate possible REE reaction paths in hydrothermal fluids. Two REE mineral deposits in New Mexico (Lemitar and Gallinas Mountains) present ideal case studies to illustrate how these models can be linked to field observations from natural systems. [1] Gysi et al. (2016), Econ. Geol. 111, 1241-1276; [2] Perry and Gysi (2020), Geochim. Cosmochim. Acta 286, 177-197; [3] Blundy and Wood (1994) Nature 372, 452-454; [4] Kulik (2006), Chem. Geol. 225, 189-212; [5] Kulik et al. (2013), Computat. Geosci. 17, 1-24. 

Monazite is a light rare earth element (REE) phosphate found in REE mineral deposits, such as those formed in (per)alkaline and carbonatite magmatic-hydrothermal systems, where it occurs in association to the development of alteration zones and hydrothermal veins. Although it has been recognized that monazite may undergo replacement by coupled dissolution-precipitation processes, currently there is no model describing the compositional REE variations in monazite resulting from direct interaction with or precipitation from hydrothermal fluids. To develop such a model requires quantification of the thermodynamic properties of the aqueous REE species and the properties of the monazite endmembers and their solid solutions. The thermodynamic properties of monazite endmembers have been determined previously using calorimetric methods and low temperature solubility studies, but only a few solubility studies have been conducted at >100 °C. In this study, the solubility products (logKs0) of LaPO4, PrPO4, NdPO4, and EuPO4 monazite endmembers have been measured at temperatures between 100 and 250 °C and saturated water vapor pressure. The solubility products are reported with an uncertainty of ±0.2 (95% confidence) according to the reaction, REEPO4(s) = REE3+ + PO43−. (see table in manuscript) The REE phosphates display a retrograde solubility, with the measured Ks0 values varying several orders of magnitude over the experimental temperature range. Discrepancies were observed between the experimental solubility products and the calculated values resulting from combining calorimetric data of monazite with the properties of the aqueous REE3+ and PO43− species available in the literature. The differences between the calculated and measured standard Gibbs energy of reaction (ΔrG0) for PrPO4, NdPO4, and EuPO4 increased with higher temperatures (up to 15 kJ mol−1 at 250 °C), whereas for LaPO4 these differences increased at lower temperatures (up to 8 kJ mol−1 at 100 °C). To reconcile these discrepancies, the standard enthalpy of formation (ΔfH0) of monazite was optimized by fitting the experimental solubility data and extrapolating these fits to reference conditions of 25 °C and 1 bar. The optimized thermodynamic data provide the first internally consistent dataset for the solubility of all the monazite endmembers, and can be used to model REE partitioning between monazite and hydrothermal fluids at >100 °C.