Average structure and microstructure of synchysite-(Ce) from Cuasso al Monte (Varese, Italy)

Abstract. Microscopic, prismatic single crystals of synchysite-(Ce) from Cuasso al Monte (typical average composition Ca1.01Ce0.37Nd0.20Y0.17La0.09Pr0.05Sm0.05Gd0.03Th0.03(CO3)2F0.72) have been investigated by a variety of techniques, including single-crystal X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopy, and precession-assisted three-dimensional electron diffraction. The synchysite crystals are affected by an extraordinary abundance of diverse defects, such as stacking faults (polytypic disorder), core–rim chemical zoning, fluid and solid (hematite) inclusions, and metamict damage. Notwithstanding these faults, reliable X-ray crystal structure refinements have been obtained, which may enrich the currently scarce database. On the other hand, electron diffraction data, while remaining within acceptable limits, evidenced some challenges in refinement, even applying dynamical theory, potentially due to the impact of defects on data quality at the nanometre scale or absorption effects in such dense compounds, even in thin foils. The apparent polytypic disorder affecting the studied synchysite actually disguises an ordered superstructure, suggesting a crystal growth by screw dislocation. The nanostructural relationships between inclusions and host suggest that the crystals trapped a large number of fluid inclusions during growth, while hematite inclusion formed later, filling voids or fractures. The studied crystals are chemically zoned, with the core enriched in Ce, La and Nd; the rim enriched in Y; and the outer rim enriched in Th. The latter, undergoing α decay, induced radiation damage to the outer rim structure of the crystals.