Mineral replacement reactions play an important role in the rare earth mineralization process of carbonatites, which reflect changes in the environmental conditions or fluid composition during the evolution of the system. Due to the close ionic size range of rare earth element ions and calcium ions, calcite can be transformed into rare earth carbonates under the action of hydrothermal solution containing rare earth element. Under the combined action of Ce³⁺ and H₂O, calcite can be gradually transformed into lanthanite [Ce₂(CO₃)₃·8 H₂O] (room temperature)→ calcioancylite [CaCe(CO₃)₂(OH)·H₂O] (70~110℃) →hydrobastnäsite (150~165℃) →cerianite (CeO₂) (200~250℃). The different binding forms of water in rare earth carbonate minerals can effectively reflect the ambient temperature. The co-participation of F^-, Ce³⁺ and H₂O can accelerate calcite to transform to rare earth minerals. Calcite can be converted into stable bastnäsite phase at room temperature, but fluorine is not easy to replace the hydroxy occupation in calcioancylite for further transformation into bastnäsite. F^-, OH^- and CO^2-₃ are important ligands responsible for the binding of rare earth elements, and play an important role in the migration, transformation and fixation of REEs in hydrothermal environment.