Dissymmetrization in Eudialyte-Group Minerals: 1. A Model of Ordered Cation Arrangement in the Crystal Structure of Amableite-(Ce) Using the P3 Symmetry

The crystal structure of a new mineral from the eudialyte group, amableite-(Ce) Na₁₅[(Ce_1.5Na_1.5)Mn₃]Mn₂Zr₃\(\square \)Si[Si₂₄O₆₉(OH)₃](OH)₂ ⋅ H₂O, was refined using X-ray diffraction analysis in the R3 space group. This mineral was discovered in a hyperagpaitic pegmatite in the Saint-Amable massif, Canada. Amableite-(Ce) is a representative of the eudialyte group with the lowest calcium content; it differs from other representatives of this group by the dominance of lanthanides in a part of edge-sharing octahedra of the six-membered ring. The structure of amableite-(Ce) has a unit cell with the following parameters: a = 14.1340(2) Å, c = 30.378(1) Å, and V = 5255.6(3) ų. In this paper, we propose a model for the distribution of cations within the amableite-(Ce) crystal structure based on a low-symmetry P3 space group. We refined 162 independent atomic positions in the isotropic–anisotropic approximation using 3968 reflections with F > 3σ(F) and obtained R = 4.6% as the final result. Despite the close agreement between the two space groups, the transition from R3 to P3 allows us to obtain a more detailed information about the local distribution of a number of elements over different framework sites. The models of the crystal structure of amableite-(Ce) in the framework of R3 and P3 symmetries, as well as other low-calcium minerals from the eudialyte group that have been studied previously within several space groups, have been compared.