Electron Microprobe Analysis of REE in Eudialyte Group Minerals: Challenges and Solutions

Alkaline complexes containing eudialyte group minerals (EGM) comprise one of the most promising sources for future rare earth element (REE) supply. Complex mineral chemistry and crystal structure of EGM pose particular challenges for resource analytics and ore characterisation. A combination of qualitative scanning electron microscope (SEM)-based image analysis and quantitative analytical methods like electron microprobe (EPMA) is required. For this purpose polished thick sections are first mapped by the mineral liberation analyzer (MLA) to effectively identify the ore-bearing minerals and mineral associations. Backscattered electron images (BSE images) indicate varying BSE contrast within the EGM crystals. In order to determine the assumed elemental variations, element distribution maps of main (e.g., Zr, Si, Al) and minor (e.g., Ce, Y) elements are obtained by EPMA. These maps illustrate variable and complex zonation patterns within the EGM crystals. Based on the element distribution maps, quantitative analyses are then performed with a field emission electron microprobe JEOL JXA 8530F.
The accurate quantification of the chemical composition of EGM is complicated by both mineralogical and X-ray-specific challenges. These include: 1) structural and chemical variability of EGM composition (e.g., [1-3]); 2) mutual interferences of X-ray lines from major and trace elements, in particular REE elements [4]; 3) the diffusive volatility of light anions as F and Cl and cations such as K and Na; 4) particular instability of EGM under the electron beam.
A novel analytical approach has been developed to account for the above mentioned analytical challenges. Additionally, loss on ignition und differential scanning calorimetry data has been applied to constrain the content and composition of volatiles in the EGM structure. All correction for the overlapping of X-ray lines is processed offline. Preliminary results demonstrate that the parameters mentioned above need to be considered and carefully optimized to perform accurate quantitative analyses on the chemical composition of EGM with the electron microprobe.