Dissymmetrization in Eudialyte-Group Minerals. II: Role of the M2-Position in Ordered P3-Model of the Structure of a Zr-Rich Member of the Eudialyte Group

A re-study of the crystal structure of low-calcium and high-zirconium eudialyte from the Khibiny Mountains of Kola Peninsula has revealed new features of its structure, which decrease the symmetry of the mineral. The ordering of Ca, Fe, Mn, and Na atoms in octahedra of six-membered rings, as well as Zr and Na atoms in M2 positions at the centers of square pyramids, with a common base, formed by the edges of these octahedra, has been established with the P3 symmetry. The trigonal unit cell parameters are a = 14.222(3) Å, c = 30.165(5) Å, and V = 5283.9 Å3. Taking into account the new data, the studied low-calcium eudialyte can be assigned to the subtype of oneillite or raslakite, being a high-zirconium variety of the latter. The study of the structure of this mineral in rhombohedral symmetry (sp. gr. R3m and R3) and trigonal Р3 symmetry has shown that the decrease in symmetry of the structural model of low-calcium representatives of the eudialyte group makes it possible to clarify the nature of ordering of the cations occupying key framework positions.