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 Maripi, Boyaca Department, Colombia, show syntaxic intergrowth of parisite-(Ce) with rontgenite-(Ce) and a phase which is assigned toB(3)S(4)(i.e., bastnasite-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) polytypeM(1)has space groupCc. Parisite-(Ce), theB(3)S(4)phase and rontgenite-(Ce) show different BSE intensities from high to low. Raman spectroscopic analyses of parisite-(Ce), theB(3)S(4)phase and rontgenite-(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 rontgenite-(Ce) by their 1092 cm(-1)/ 1081 cm(-1)nu(1)(CO3) band height ratio.