The thermodynamic properties of bastnäsite-(Ce) and parisite-(Ce)

- We have investigated the thermodynamic properties of REE fluorocarbonate minerals.
- The isobaric heat capacity (Cp°) of bastnäsite-(Ce) and parisite-(Ce) was measured as function of temperature.
- Bastnäsite-(Ce) and parisite-(Ce) decompose irreversibly at 1 bar and temperatures of 824.2 and 842.7 K, respectively.
- The enthalpy of formation at 298 K and 1 bar was determined from decomposition enthalpies, and entropy was estimated.
- The data permits to calculate mineral-fluid equilibria at P-T-x conditions relevant for the formation of REE deposits.

The rare earth elements (REE) are critical metals that play a major role in emerging high technology and green industries. The light (L)REE occur dominantly in fluorocarbonate minerals and consequently information on the stability of these minerals is essential for a better understanding of the genesis of REE deposits and for the efficient processing of their ores. We have investigated the thermochemical properties of natural bastnäsite-(Ce) (Ce0.50La0.25Nd0.20Pr0.05CO3F) and parisite-(Ce) (CaCe0.95La0.60Nd0.35Pr0.10(CO3)3 F2) using differential scanning calorimetry at temperatures from 323 K to 1022 K and a pressure of 1 bar employing heat ramping and isothermal methods. Crystal lattice parameters of the REE fluorocarbonate minerals and reaction products from the experiments were determined using X-ray diffraction. The measured isobaric heat capacity (Cp°) for bastnäsite-(Ce) can be described by the relationship 134.3 - 2.032 × 106 T- 2 between 343.15 and 528.15 K and for parisite-(Ce) by the relationship 398.8 - 1048 T− 0.5 - 4.202 × 106 T− 2 between 343.15 and 643.15 K, where T is temperature in K. Bastnäsite-(Ce) decomposed irreversibly at > 612 K to form REE oxyfluorides and CO2. An endothermic peak at 824.2 K yielded a heat of reaction of 245.2 ± 2.5 kJ/mol. Parisite-(Ce) decomposed irreversibly at > 664 K to form REE oxyfluorides, CaCO3 and CO2. An endothermic peak at 842.7 K yielded a heat of reaction of 522.6 ± 5.2 kJ/mol. The enthalpy of formation at 298 K and 1 bar was retrieved from the decomposition enthalpies, yielding − 1808.4 ± 12.0 kJ/mol and − 4848.0 ± 23.8 kJ/mol for bastnäsite-(Ce) and parisite-(Ce), respectively. The measured molar volumes for bastnäsite-(Ce) and parisite-(Ce) are 42.91 cm3/mol and 122.71 cm3/mol, respectively. An estimation method, based on the dependence of entropy on volume, was used to retrieve the third law entropy (S°) at 298.15 K, and together with the measured thermodynamic properties, permitted us to construct the first quantitative mineral-fluid stability diagrams involving bastnäsite-(Ce), parisite-(Ce), fluocerite-(Ce), calcite and fluorite at P-T-x conditions relevant for the study of natural Ca-REE-C-O-H-F systems. Further studies of the thermodynamic properties of REE-bearing minerals are urgently needed to better understand the genesis of REE ore deposits.