Crystallisation pathways of a mixed La-Nd carbonate – Ambient temperature synthesis of the mineral lanthanite

Naturally occurring rare earth element (REE) bearing carbonate minerals contain different REEs, yet efforts to synthesize and understand their formation mechanisms have hitherto used single REE end-member experiments. In synthesising a mixed Lasingle bondNd carbonate, we document how the presence of two REEs affects the overall crystallisation process. We find that the mixed REE carbonate has a broadly similar crystallisation pathway as its end-members but with key differences. The process begins with the precipitation of amorphous spherical nanoparticles which mature into macrocrystals via an intermediary polymorphous phase. The resulting phase is identified as lanthanite, a naturally occurring REE carbonate mineral. The mixed carbonate has unit cell parameters and phase transition times in between that of its La and Nd end-members. Unlike the end-members, the crystallisation process of mixed REE carbonates has a consistent two-stage transition process between the nanoparticulate and the final phase. They also have a distinct and homogeneous morphology compared to their end-members. These results highlight the importance of considering multiple REEs simultaneously when studying the precipitation mechanisms of REE carbonates.