Thermodynamic Stability of Hydrated Rare Earth Carbonates (Lanthanites)

The mineral lanthanite, REE₂(CO₃)₃·xH₂O (x ∼ 4 ≤ x ≤ 8) (REE = rare earth element), is important in low-temperature geological settings, and understanding REE fractionation and separation is essential in both mineralogical and technological contexts. However, a lack of stability data has limited the development of thermodynamic models in this system. This study addresses this deficiency by using room-temperature solution calorimetry and low-temperature heat capacity measurements and empirical correlations to provide new thermochemical data for lanthanites (from La to Lu). The measurements are combined with empirical fitting and extrapolation to determine thermodynamic constraints on variation in H₂O content within the lanthanite system. Systematic analysis reveals a significant decrease in stability with increasing ionic radius, which is consistent with strong fractionation between the light and heavy REEs. This fractionation is up to ten times larger for lanthanites than for other important REE compounds such as phosphates, indicating potential for REE fractionation, and this has major implications for REE fractionation in both natural systems and industrial REE separation processes.