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. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s00710-020-00723-x) contains supplementary material, which is available to authorized users.