Crystal structure of calcioburbankite and the characteristic features of the burbankite structure type

The crystal structure of calcioburbankite (Na,Ca)3(Ca,RE,Sr,Ba)3(CO3)5 found in carbonatites from Vuoriyarvi (North Kareliya) was solved by the Rietveld method. The experimental data were collected on an ADP-2 diffractometer (λCuKα radiation; Ni filter; 16.00° < 2θ < 130.00°; the number of (α1 + α2) reflections was 455). All the calculations were performed within the sp. gr. P63mc; a = 10.4974(1) Å, c = 6.4309(1) Å, V = 613.72(1) Å3; Rwp = 2.49%. The structure was refined with the use of the anisotropic thermal parameters for the (Na,Ca) and (Sr,Ba,Ce) cations. The comparison of the crystal structures of all of the known hexagonal representatives of the burbankite family demonstrates that the burbankite structure type (sp. gr. P63mc) is stable, irrespectively of the occupancy of the ten-vertex polyhedra predominantly with Ca, Sr, or Ba cations and the occupancies of the positions in the eight-vertex polyhedra.