CRYSTAL CHEMISTRY AND PHYSICAL-CHEMICAL BEHAVIOR OF REE-BEARING PHOSPHATES AND ARSENATES: THE CASE STUDY OF MT. CERVANDONE

The crystal chemistry, crystal structure and behavior at non-ambient conditions of natural Rare Earth Elements-bearing phosphates [monazite-(Ce), nominally Ce(PO4), and xenotime-(Y), nominally Y(PO4)] and arsenates [gasparite-(Ce), nominally Ce(AsO4), and chernovite-(Y), nominally Y(AsO4)], from the hydrothermal quartz-bearing fissures outcropping at Mt. Cervandone (Lepontine Alps, Piedmont, Italy), have been investigated. The chemical and structural characterization, performed via electron microprobe analysis, Raman spectroscopy and single-crystal X-ray diffraction, showed that the zircon-type minerals chernovite-(Y) and xenotime-(Y) share a very similar (Y,HREE) composition and the same can be stated for the LREE content of the two monazite-type minerals [gasparite-(Ce) and monazite-(Ce)]. An almost complete solid solution has been found between xenotime-(Y) and chernovite-(Y), while a wide miscibility gap has been observed within the monazite series minerals of Mt. Cervandone. Moreover, both the unit-cell and the REE-coordination polyhedral volumes are strongly controlled by the cationic population at the T-site: an increase in As not only expands the volume of the TO4 tetrahedron, but even that of the REE-polyhedron, irrespective of the A-site population. An exception is provided by the relative abundance of Th and Ca at the A-site, which was found to expand the coordination polyhedron and unit-cell volumes irrespective of the T-site composition. The comparative analysis of the thermo-elastic behavior of selected samples has been conducted by in situ high-P, high-T and combined HP–HT X-ray diffraction experiments using both single-crystals and polycrystalline samples at synchrotron or conventional lab facilities. The compressional behavior of the investigated minerals is compatible with the literature data on the synthetic endmember counterparts. The two zircon-type minerals undergo different phase transitions under compression, while the monazite-type minerals experience a change in the compressional behavior, likely induced by the increase in coordination number of the REE-bearing polyhedron. The thermal expansion behavior of monazite-(Ce) and xenotime-(Y) confirm the literature data available, while the studied chernovite-(Y) is significantly less expandable than the synthetic counterparts described in the literature. The phosphates were found to be less compressible, but more expandable than the isostructural arsenates. The structural analysis at non-ambient conditions has been carried out on the basis of the refined structure models, allowing the description of the deformation mechanisms accommodating the bulk compression or expansion at the atomic scale. Both are mainly accommodated by the REEOx coordination polyhedra, while the tetrahedra behave as a quasi-rigid units. In conclusion, all the experimental data confirm the central role played by the T-site in controlling the structural deformation and, in turn, the bulk thermal expansion and compression.