Dissymmetrization in eudialyte-group minerals. IV. Features of the blocky isomorphism in the structure of the Nb-deficient oneillite analogue in the frame of P3-symmetry

The crystal structure of a potentially new member of the eudialyte group, the Nb-deficient analogue of oneillite from Mont Saint-Hilaire, Québec, Canada, with the idealized formula: Na13(Ca3Mn3)Zr3(Fe,Mn)3( ,Nb)(Si,Nb, ) [Si3O9]2[Si9O27]2(O,OH,Cl)3•2H2O, has been re-studied within the space group P3. The unit-cell parameters are: a = 14.134(3), c = 30.178(6) Å, V = 52201 Å3. The crystal structure of the mineral has been previously investigated using the space group R3 (typical for the members of oneillite family). In this work a structural model characterized by 155 crystallographic sites was solved in the frame of the low symmetry space group P3 and refined to R = 5.9 % using 4179 reflections with F > 2 (F). The cation distribution between the key sites in low symmetry is more detailed.