The crystal structure of parisite-(Ce), Ce2CaF2(CO3)(3)

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 contra st to the putative hexagonal cell, weak XRD maxima on precession films demo nstrate that the parisite-(Ce) is monoclinic with a space group C2/c Or CC. The unit cell was refined as a = 12.305(2), b = 7.1053(5), c = 28.250(5) An gstrom, and beta = 98.257(14)degrees 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 carb onate groups. The [001] layer stacking sequence is... (Ca), (CO3), (CeF), ( CO3), (CeF), (CO3), (Ca), (CO3), (CeF), (CO3), (CeF), (CO3).... The parisite-(Ce) structure can be considered as two portions of the bastna site structure connected by a (Ca) layer. The insertion of a (Ca) layer wou ld create long [001]*: structure voids in the F columns if the bastnasite p ortions stacked hexagonally; therefore, the two bastnasite portions are off set along [(1) over bar 10] by a/6(sin60 degrees) (or [(1) over bar 00] in hexagonal cells, by a/3) such that the oxygen atoms of the vertical edges o f the carbonate groups occupy the voids. Polytypism results because a bastn asite portion can shift equally in two possible directions, +/-120 degrees from the previous offset vector. The parisite-(Ce) structure elucidated her ein 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 du ring our extensive study.