Dissymmetrization in eudialyte-group minerals. I. A model of ordered cation arrangement in the crystal structure of amableite-(Ce) using the P3 symmetry

The crystal structure of the recently discovered eudialyte group mineral, amabellite-(Ce) Na15[(Ce1.5Na1.5)Mn3]Mn2Zr3£Si[Si24O69(OH)3](OH)2 · H2O, found in the hyperagpaitic pegmatite of the Saint-Amable massif (Canada), has been solved by X-ray structural analysis within the space group R3. Amabellite-(Ce) is a member of the eudialyte group with the lowest calcium content and differs from other members of this group by the dominance of lanthanides in the part of the edge-sharing octahedra of the six-membered ring. The unit cell parameters of the mineral are: a = 14.1340(2), c = 30.378(1) Å, V = 5255.6(3) ų. A model of the cation distribution in the crystal structure of amabellite-(Ce) within the low-symmetry space group P3 has been proposed. The obtained 162 independent positions were refined in the isotropic-anisotropic approximation of atomic displacements using 3968 F 3σ(F), R = 4.6%. Despite the fairly close results, the transition from space group R3 to P3 allows for more detailed information on the local distribution of a number of elements over the framework positions. A comparison was made between the crystal structure models of amabellite within the symmetries R3 and P3, as well as other low-calcium eudialyte group minerals previously studied within several space groups.