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

Abstract

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)3F2) using differential scanning calorimetry at temperatures from 323K to 1022K and a pressure of 1bar 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 134.3–2.032×106T-2 between 343.15 and 528.15K and for parisite-(Ce) by the relationship 398.8–1048T−0.5–4.202×106T−2 between 343.15 and 643.15K, where T is temperature in K. Bastnäsite-(Ce) decomposed irreversibly at >612K to form REE oxyfluorides and CO2. An endothermic peak at 824.2K yielded a heat of reaction of 245.2±2.5kJ/mol. Parisite-(Ce) decomposed irreversibly at >664K to form REE oxyfluorides, CaCO3 and CO2. An endothermic peak at 842.7K yielded a heat of reaction of 522.6±5.2kJ/mol. The enthalpy of formation at 298K and 1bar was retrieved from the decomposition enthalpies, yielding −1808.4±12.0kJ/mol and −4848.0±23.8kJ/mol for bastnäsite-(Ce) and parisite-(Ce), respectively. The measured molar volumes for bastnäsite-(Ce) and parisite-(Ce) are 42.91cm3/mol and 122.71cm3/mol, respectively. An estimation method, based on the dependence of entropy on volume, was used to retrieve the third law entropy (S°) at 298.15K, 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.