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

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