Crystal structure of a new Mn, Na-ordered analogue of eudialyte with the symmetry R3

The crystal structure of a new representative of the eudialyte family was s tudied. This mineral is characterized by a low calcium content and by order ing the elements that isomorphically replace calcium, which lowers the symm etry from R3m to R3. The structure of the sample under study consists of th e six-membered rings with two octahedra of substantially different volumes, one occupied mainly by manganese and the other, mainly by sodium and chara cterized by the average distances of 2.22 and 2.45 Angstrom, respectively. The formation of such octahedra is the most characteristic structural featu re of the third low-symmetry mineral of the eudialyte family. (C) 2000 MIAI K "Nauka/Interperiodica".