High-pressure behavior and crystal chemistry of natural REE-bearing arsenates and phosphates

Rare Earth Elements (REE) are a group of 17 elements, containing the lanthanides and their geochemical cognates Sc and Y. REEs are fundamental elements in several high-tech applications, including magnets, catalysis and glass additives. All the REEs have been classified as critical raw materials, for their elevated supply risk. Despite their high economic relevance, only few studies have been devoted to the crystal chemistry, behavior and phase stability at non-ambient conditions of natural REE-bearing minerals, characterized by a multi-REE composition. The REE-composition seems to highly influence the stability field of REE-bearing compounds [1] and, in this light, we aim to study the high-P phase stability and elastic behavior of four natural multi-REE-bearing minerals, occurring at the Mt. Cervandone deposit (Piedmont, Italy): chernovite-(Y), (nominally YAsO4), gasparite-(Ce) (nominally CeAsO4), xenotime-(Y) (nominally YPO4) and monazite-(Ce) (nominally CePO4). Chernovite-(Y) and xenotime-(Y) share the same zircon-type structure with tetragonal symmetry (space group I41/amd) and are characterized by HREE- (Gd-Lu series) and Y-enrichment. Conversely, gasparite-(Ce) and monazite-(Ce) share the same LREE (La-Eu) enrichment and the so-called monazite-type structure (space group P21/n). All the selected minerals have been investigated over 20 GPa, using a diamond anvil cell (DAC), by means of in situ synchrotron single-crystal X-ray diffraction. For both the zircon-type structure minerals, a P-induced phase transition has been observed: chernovite-(Y) undergoes a transition to a scheelite-type structure at ~11 GPa, as previously observed for its synthetic pure analogue YAsO4 [2]. The phase transition in xenotime-(Y) to a monazite-type structure, as for synthetic pure YPO4, is observed between 17 and 19 GPa [3]. However, synthetic YPO4 is characterized by a second phase transition to a scheelite-type structure [3], but in our natural studied sample a similar behavior has not been observed at least up to 30 GPa. Conversely, for gasparite-(Ce) and monazite-(Ce), no phase transitions occur within the pressure-range under investigation. For all the investigated REE-bearing minerals, both with tetragonal and monoclinic symmetry, the bulk compression is mainly accommodated by the REE-coordination polyhedra. Whereas in the tetragonal chernovite-(Y) and xenotime-(Y) the [001]-axis is the less compressible direction, due to the polyhedral chains running along this axis, in gasparite-(Ce) and monazite-(Ce) the distortion of the REEO9 polyhedra leads to the lowest linear compression laying in the (010) plane and, therefore, not coinciding with the direction of chain development. The structural analysis also reveals that the arsenates are more compressible with respect to the isostructural phosphates. The observed difference in compressibility between phosphates and arsenates is related to the different behavior of the As and P coordination tetrahedra: AsO4 shows a significant compression, especially in the low-P regime, whereas PO4 substantially behaves as a rigid body under pressure. [1] Lacomba-Perales, R., Errandonea, D., Meng, Y., & Bettinelli, M. Phys. Rev. 2010 B81, 064113. [2] Errandonea, D., Kumar, R., Lopez-Solano, J., Rodriguez-Hernandez, P., Muñoz, A., Rabie, M. G., & Puche, R. S. Phys. Rev. 2011 B83, 134109. [3] Zhang, F. X., Wang, J. W., Lang, M., Zhang, J. M., Ewing, R. C., & Boatner, L. A. Phys. Rev. 2009 B80, 184114.