The parisite–(Ce) enigma: challenges in the identification of fluorcarbonate minerals

A multi-methodological study was conducted in order to provide further insight into the structural and compositional complexity of rare earth element (REE) fluorcarbonates, with particular attention to their correct assignment to a mineral species. Polycrystals from La Pita Mine, Municipality de Maripí, Boyacá Department, Colombia, show syntaxic intergrowth of parisite–(Ce) with röntgenite–(Ce) and a phase which is assigned to B 3 S 4 (i.e., bastnäsite-3–synchisite-4; still unnamed) fluorcarbonate. Transmission electron microscope (TEM) images reveal well-ordered stacking patterns of two monoclinic polytypes of parisite–(Ce) as well as heavily disordered layer sequences with varying lattice fringe spacings. The crystal structure refinement from single crystal X-ray diffraction data – impeded by twinning, complex stacking patterns, sequential and compositional faults – indicates that the dominant parisite–(Ce) polytype M 1 has space group Cc . Parisite–(Ce), the B 3 S 4 phase and röntgenite–(Ce) show different BSE intensities from high to low. Raman spectroscopic analyses of parisite–(Ce), the B 3 S 4 phase and röntgenite–(Ce) reveal different intensity ratios of the three symmetric CO 3 stretching bands at around 1100 cm −1 . We propose to non-destructively differentiate parisite–(Ce) and röntgenite–(Ce) by their 1092 cm −1 / 1081 cm −1 ν 1 (CO 3 ) band height ratio.