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

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 bastnasite-(Ce) (Ce0.50La0.25Nd0.20Pr0.05CO3F) and parisite-(Ce) (CaCe0.95La0.60Nd0.35Pr0.10(CO3)(3) F-2) 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 (C-p degrees) for bastnasite-(Ce) can be described by the relationship 134.3-2.032 x 10 (6) T-2 between 343.15 and 528.15 K and for parisite-(Ce) by the relationship 398.8 - 1048 T-0.5 -4.202 x 10 (6) T-2 between 343.15 and 643.15 K, where T is temperature in K. Bastnasite-(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 bastnsite-(Ce) and parisite-(Ce), respectively. The measured molar volumes for bastnasite-(Ce) and parisite-(Ce) are 42.91 cm(3)/mol and 122.71 cm(3)/mol, respectively. An estimation method, based on the dependence of entropy on volume, was used to retrieve the third lawentropy (S degrees) at 298.15 K, and together with the measured thermodynamic properties, permitted us to construct the first quantitativemineral-fluid stability diagrams involving bastnasite-(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. (C) 2014 Elsevier B.V. All rights reserved.