High-pressure crystal chemistry of four natural REE(As,P)O4 minerals from Mt. Cervandone, Italy

REE orthoarsenates and orthophosphates are common accessory minerals characterized by the general chemical formula REEXO4, where REE represents one of the lanthanides (La-Lu series), Y, Sc, Ca or Th, whereas X stands for As, P or Si. In the framework of a long-term project on the high-T/high-P crystal-chemistry and phase-stability of REE-bearing minerals, the high-pressure behavior of chernovite-(Y) (nominally YAsO4), xenotime-(Y) (nominally YPO4) gasparite-(Ce) (nominally CeAsO4) and monazite-(Ce) (nominally CePO4), has been studied. Chernovite-(Y) and xenotime-(Y) show a HREE- (Gd-Lu series) and Y-enrichment, and the same tetragonal symmetry (space group I41/amd), whereas gasparite-(Ce) and monazite-(Ce) share the same LREE (La-Eu) enrichment and monoclinic cell (space group P21/n). All these minerals occur at Mt. Cervandone (Western Alps, Italy), a renowned Alpine REE-bearing mineral deposit. The crystal chemistry of the four minerals has been studied via EPM-WDS analysis. Excluding gasparite-(Ce), which formation is bound to the replacement of the mineral synchisite-(Ce) (CaCe(CO3)2F), a sensible enrichment in Gd and Ho is observed. Moreover, the majority of the chernovite-(Y) show a variable amount of ThO2, up to 13 wt%, and phosphorous as substitute for arsenic in almost every proportion. In the case of the monoclinic series between monazite-(Ce) and gaparite-(Ce), no solid solution has been observed. Experiments at high-pressure were performed by in situ synchrotron X-ray diffraction using a diamond anvil cell. The high-pressure behavior of single crystals of xenotime-(Y), gasparite-(Ce) and monazite-(Ce) has been studied up to ~20 GPa, whereas that of chernovite-(Y) has been studied by powder diffraction up to 8.20(5) GPa. A II-order Birch-Murnaghan equation of state was fitted to the V-P data, within the phase stability field of the minerals, yielding the following bulk moduli: KP0,T0 = 125(3) GPa (βV0 = 0.0080(2) GPa-1) for chernovite-(Y); KP0,T0 = 145(2) GPa (βV0 = 0.0069(1) GPa-1) for xenotime-(Y); KP0,T0 = 106.7(9) GPa (βV0 = 0.0094(1) GPa-1) for gasparite-(Ce), KP0,T0 = 121(2) GPa (βV0 = 0.0083(1) GPa-1) for monazite-(Ce). K’ = ∂KV/∂P = 4 (fixed) for all the minerals. Deformation mechanisms, at the atomic scale, were described on the basis of structure refinements. Acknowledgments: This research was partly funded by the PRIN2017 project “Mineral reactivity, a key to understand large-scale processes” (2017L83S77).