Desymmetrization in eudialyte-group minerals. II. A role of м2-position in ordered р3-model of zr-rich member eudialyte group structure

A re-study of the crystal structure of low-calcium and high-zirconium eudialyte from the Khibiny Mountains of the Kola Peninsula has revealed new features of its structure, namely a decrease in the symmetry of the mineral. The ordering of the Ca, Fe, Mn and Na atoms within the octahedrons of six-membered rings, as well as the Zr and Na atoms in the M2 positions at the centre of the square pyramids, was established within the framework of P3 symmetry. The trigonal cell parameters are a = 14.222(3), c = 30.165(5) Å, and V = 5283.9 ų. In light of the new data, the low-calcium eudialyte can be classified as either oneillite or raslakite, specifically a high-zirconium variety of the latter. The study of the structure of this mineral in rhombohedral symmetry (space group R3m and R3) and trigonal P3-symmetry has demonstrated that the reduction in structural symmetry of the low-calcium representatives of the eudialyte group allows for the clarification of the ordering of cations occupying pivotal positions within the framework.