High-pressure behavior of REE-bearing phosphates and arsenates

ATO4 compounds (A= Sc, Y, Ln, U and Th; T stands for tetrahedrally-coordinated cations, e.g. P and As), represent a wide class of minerals, which includes the REE-bearing arsenates chernovite-(Y) (YAsO4) and gasparite-(Ce) (CeAsO4), and the more common REE-bearing phosphates, xenotime-(Y) (YPO4) and monazite-(Ce) (CePO4). Chernovite-(Y) and xenotime-(Y) share the same HREE-enriched, zircon-type structure (I41/amd), whereas the LREE-enriched gasparite-(Ce) and monazite-(Ce) crystallize in the so-called monazite-type structure (P21/n). The HP behavior of the REETO4 compounds has been object of many studies, mainly focused on their synthetic counterparts. In this work, we have studied the HP and combined HP–HT behavior of natural samples of the abovementioned minerals, using in situ single-crystal synchrotron X-ray diffraction. A special attention was devoted to the relationships between chemical and structural features at non-ambient conditions. In particular, the compressional behavior of the REE-polyhedron, T-site tetrahedron and the deformation mechanisms acting at the atomic scale, poorly studied in the current literature, have been described and discussed. For both the arsenates and phosphates, the monazite-type minerals are found to be more compressible than the zircon-type ones, and the arsenates more compressible than the phosphate analogues. The analysis of the refined structure models showed that the T-tetrahedron is almost uncompressible and the nature of its dominant cation (As or P) significantly affects the response to (T,P)-stimuli of the A-polyhedron.