Synchysite-(Ce) from Cinquevalli (Trento, Italy): Stacking Disorder and the Polytypism of (Ca,REE)-Fluorcarbonates

Synchysite-(Ce) at Cinquevalli occurs as fine needles intergrown with quartz in quartz-dikes and in association with altered K-feldspar and oxidized chalcopyrite as major constituents. Synchysite-(Ce) [Ca1.00(Ce0.43La0.26Nd0.17Y0.07Pr0.04Sm0.02Gd0.01)Σ=1.00(CO3)2(F0.58(OH)0.42)], shows an overgrowth rim of bastnäsite-(Ce) [(Ce0.34La0.25Nd0.17Pb0.07C a0.06Y0.06Pr0.04S m0.02Gd0.01)Σ=1.00C O3(F0.75(OH)0.25)]. Unit cell refinement of synchysite (C2/c) and bastnäsite (P62c) led to a = 12.272(4), b = 7.100(2), c = 18.640(5) Å, β = 102.71(5)°, and a = 7.085(1), c = 9.746(2) Å, respectively. Polysomatic faults are sporadic, but polytypic disorder is widespread. High resolution transmission electron microscopy images taken along [100] or ⟨130⟩ show an apparent order and the related diffraction patterns are streak-free. Conversely, along [010] or ⟨110⟩, a high density of stacking faults is observed and the related diffraction patterns show hhl rows with h ≠ 3n affected by streaks. No ordered domain larger than a few unit cells was detected. The stacking sequence of (Ca,REE)-fluorcarbonates can be compared with subfamily-B mica polytypes (2M2, 2O and 6H), which are characterized by n·60° (n = odd) rotations. Subfamily-A polytypes (1M, 2M1 and 3T), characterized by n·60° (n = even) rotations, should not be possible. Synchysite, characterized by ±60° rotations, can be likened to the 2M2 polytype.