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: K_P0,T0 = 125(3) GPa (β_V0 = 0.0080(2) GPa^-1) for chernovite-(Y); K_P0,T0 = 145(2) GPa (β_V0 = 0.0069(1) GPa^-1) for xenotime-(Y);  K_P0,T0 = 106.7(9) GPa (β_V0 = 0.0094(1) GPa^-1) for gasparite-(Ce), K_P0,T0 = 121(2) GPa (β_V0 = 0.0083(1) GPa^-1) for monazite-(Ce). K’ = ∂K_V/∂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).