Comparative thermal and compressional behaviour of natural xenotime-(Y), chernovite-(Y) and monazite-(Ce)

ATO4 compounds are a class of oxides which includes, among others, the rare-earth elements bearing phosphates and arsenates, REEPO4 and REEAsO4. In this study, we have investigated the isothermal high-pressure and the isobaric high-temperature behaviour of natural samples of xenotime-(Y) (ideally YPO4), chernovite-(Y) (YAsO4) and monazite-(Ce) (CePO4) from the hydrothermal veins outcropping at Mt. Cervandone (Western Italian Alps). Experimental data based on in situ X-ray diffraction (both single-crystal and powder techniques with conventional or synchrotron radiation) allow to fit the unit-cell volumes and axial thermal and compressional evolution and provide a suite of refined thermo-elastic parameters. A comprehensive analysis of the role played by the crystal chemistry on the thermo-elastic response of these minerals is discussed, along with the description of the main crystal-structural deformation mechanisms for both the zircon (xenotime and chernovite) and monazite (monazite) structural types. Pressure-induced phase transitions of xenotime-(Y) and chernovite-(Y) are discussed and compared with previous literature data, whereas a change in the compressional behaviour of monazite-(Ce) at ~ 18 GPa, involving an increase in the coordination number of the REE-hosting A site, is presented and discussed.