Parisite-(Ce)

Abstract: The crystal structure of parisite–(Ce) was solved and refined to R = 0.044, Rw = 0.037 using three–dimensional X–ray diffraction data (XRD). In contrast to the putative hexagonal cell, weak XRD maxima on precession films demonstrate that the parisite–(Ce) is monoclinic with a space group C 2/ c or Cc . The unit cell was refined as a = 12.305(2), b = 7.1053(5), c = 28.250(5) A, and β = 98.257(14)°. The structure refinement confirmed the space group Cc . Like bastnasite and synchysite, parisite possesses a (001) layer structure, with layers of (Ca) and (CeF) separated by layers of carbonate groups. The [001] layer stacking sequence is ... (Ca), (CO 3 ), (CeF), (CO 3 ), (CeF), (CO 3 ), (Ca), (CO 3 ), (CeF), (CO 3 ), (CeF), (CO 3 ).... The parisite-(Ce) structure can be considered as two portions of the bastnasite structure connected by a (Ca) layer. The insertion of a (Ca) layer would create long [001]* structure voids in the F columns if the bastnasite portions stacked hexagonally; therefore, the two bastnasite portions are offset along [110] by a /6(sin60°) (or [1 − 00] in hexagonal cells, by a /3) such that the oxygen atoms of the vertical edges of the carbonate groups occupy the voids. Polytypism results because a bastnasite portion can shift equally in two possible directions, ±120° from the previous offset vector. The parisite-(Ce) structure elucidated herein is the simplest and most common polytype possible, 2M. The polytype 6R, which was previously believed to be the most common, has not been found during our extensive study.

Abstract: 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 to B3S4 (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) polytype M1 has space group Cc. Parisite–(Ce), the B3S4 phase and rontgenite–(Ce) show different BSE intensities from high to low. Raman spectroscopic analyses of parisite–(Ce), the B3S4 phase and rontgenite–(Ce) reveal different intensity ratios of the three symmetric CO3 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 ν1(CO3) band height ratio.