Nanoscale intergrowths in the bastnäsite–synchysite series record transition toward thermodynamic equilibrium

Abstract The accelerated pace of transition to green energy technologies has led increased demand for rare-earth elements (REEs). Fluorocarbonates from the bastnäsite [REE(CO 3 )F]—synchysite [CaREE(CO ) 2 F] group are abundant in nature and dominant REE minerals >10 billion ton Olympic Dam Cu–U–Au–Ag deposit, South Australia. Intergrowths synchysite slabs at nanometer micron scales rule rather than exception account structural compositional changes between two endmembers. This article reviews REE-fluorocarbonate mineralogy with emphasis on significance intergrowths among achievement thermodynamic equilibrium. Imaging by high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) allows visualization atomic-scale periodicity modularity fluorocarbonates and, turn, determination domain compositions, which otherwise below spatial resolution microbeam methods. Despite highly irregular, such domains featuring can approach compositions next intermediate member phase within each host, implying formation a system close Graphic