Quantitative Determination of Calcium Rare Earth Fluoro-carbonate Minerals by Electron Probe Microanalyzer

Rare earth fluoro-carbonate minerals are the most important LREE-bearing minerals in industrial development. Accurate determination of the chemical composition of calcium rare earth fluoro-carbonates is a prerequisite for rapid identification of their species and an essential requirement for studying the genesis of rare earth deposits and the efficient development and utilization of rare earth resources. In this study, by optimizing the experimental conditions, direct determination of carbon content with a subsequent calibration, we have improved the quantitative analytical method for accurately analyzing the chemical composition of calcium rare earth fluoro-carbonate minerals using an EPMA. The calibration calculation of carbon content in the samples was performed using both theoretical crystallochemical formula and charge balance methods, and the results showed that the theoretical crystallochemical formula method yielded more reasonable results. Based on compositional analysis by EPMA and structural analysis by Laser Raman Spectroscopy, Jinchuan syenogranite comprises at least two species of calcium rare earth fluoro-carbonate minerals. The predominant phase is synchysite, the minor phase that exhibited as radial needles is parasite or roentgenite-(Ce). Based on the mineral paragenesis, it is believed that the calcium rare earth fluoro-carbonate minerals in the Jinchuan syenogranite are formed during the post-magmatic stage due to the metasomatism of apatite by F⁻, CO₂, and REE³⁺-rich fluids.