High-pressure behavior of REE-bearing arsenates and phosphates from the hydrothermal fissures of Mt. Cervandone, Western Alps, Italy

ATO4 minerals, where A= Sc, Y, REE, U and Th, whereas T stands for tetrahedrally-coordinated cations (i.e., As, P, Si), represent a large group of common accessory minerals in hydrothermal alteration of granitoid rocks. These include the REE-bearing arsenates chernovite-(Y) (YAsO4) and gasparite-(Ce) (CeAsO4), as well as the more common REE-bearing phosphates, xenotime-(Y) (YPO4) and monazite-(Ce) (CePO4). Chernovite-(Y) and xenotime-(Y) share the same zircon-type structure with tetragonal symmetry (I41/amd), whereas gasparite-(Ce) and monazite-(Ce) are characterized by the so-called monazite-type structure and monoclinic unit-cell (P21/n). Although fairly rare, in the last decade, the REE-bearing arsenates have been detected in several sites and their role in REE partitioning might be important. The Mt. Cervandone (Piedmont, Italy) mineral deposit is one of the most renowned within the Alps. In this study, we report the results on the crystal chemistry and high-P behavior of the four above-mentioned mineral species, obtained by EPM-WDS chemical analysis and in situ synchrotron XRD at high-pressure exceeding 20 GPa, using a diamond anvil cell. The chemical analysis reveals a nearly complete solid solution between chernovite-(Y) and xenotime-(Y), whereas a miscibility gap apparently occurs between gasparite-(Ce) and monazite-(Ce). Moreover, the presence of a complex solid solution between the isostructural chernovite-(Y), xenotime-(Y) and thorite, up to ~13 mol % of ThSiO4 has been observed. The high-pressure behavior of chernovite-(Y), xenotime-(Y), gasparite-(Ce), monazite-(Ce) and Th-rich chernovite-(Y) has been studied. For the zircon-type structure minerals, a P-induced phase transition has been detected, whereas the monazite-type compounds are stable within the whole pressure-range investigated. Based on the P-V data, the isothermal bulk moduli of the five selected minerals (two samples of chernvite-(Y)) have been refined, using a 2-order Birch-Murnaghan EoS: 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=105(1) GPa (βV0=0.0095(1) GPa-1) for gasparite-(Ce), KP0,T0 =121(2) GPa (βV0=0.0083(1) GPa-1) for monazite-(Ce) and KP0,T0=123.8(9) GPa (βV0=0.00808(6) GPa-1) for Th-rich chernovite-(Y).. The monazite-type structures are found to be always more compressible with respect to the zircon-type structures. Moreover, within each structure-type, the arsenates are always more compressible than the isostructural phosphates. Although the REE-coordination polyhedron accommodates most of the bulk compression, the tetra-coordinated cation (As or P) controls the different compressibility of arsenates and phosphates: PO4 tetrahedra acts as rigid bodies under pressure, whereas AsO4 is characterized by a significant compression, especially in the low-P regime. No significant differences have been observed among the high-pressure behaviors of Th-poor and Th-rich chernovite-(Y).