Reaction Pathways toward the Formation of Bastnäsite: Replacement of Calcite by Rare Earth Carbonates

Abstract

The interaction of rare earth bearing (La, Nd, Dy) aqueous solutions with calcite crystals at ambient and hydrothermal conditions (25–220 °C) results in the solvent-mediated surface precipitation and subsequent pseudomorphic mineral replacement of calcite by rare earth carbonates. The newly formed rare earth carbonates follow the crystallization sequence lanthanite [REE2(CO3)3·8H2O] → kozoite [orthorhombic REECO3(OH)] → hydroxylbastnäsite [hexagonal REECO3(OH)]. The specific rare earth involved in these processes and the temperature have strong control of the polymorph selection, crystallization pathways, and kinetics of mineral replacement. Kozoite-(La, Nd) grows oriented onto the calcite surface, forming an epitaxy. This phase forms elongated crystals on [100], with the {011} and {01̅1} as major forms. The epitaxial relationship is (104) [010]cc∥(001) [100]koz and is strongly dependent on the ionic radius of the rare earth in the structure of kozoite.