Dissymmetrization in Eudialyte-Group Minerals. III: Peculiarities of the High-Order Voronkovite Р3-Structure

The re-study of the crystal structure of the second find of low-calcium and rare-earths bearing voronkovite in the Lovozero alkaline complex, Kola Peninsula has revealed new features of its structure, which lower the symmetry of the mineral. Ordering of Ca, Mn, Ce, and Na atoms in octahedra of six-membered rings, as well as Zr, Fe, and Na atoms in M2 positions, combining translationally identical six-membered rings as well as Nb, Ti, and Si atoms and vacancies in axial fragments of the structure, has been established within the P3 symmetry. The trigonal-cell parameters of the mineral are a = 14.1617(1) Å, c = 30.1815(1) Å, and V = 5242.09(4) Å3; the number of independent positions is 184. The crystal structure has been refined to the R factor of 3.74% in the isotropic atomic displacement approximation using 8257 reflections with F > 3σ(F). The main features of voronkovite in comparison with the minerals investigated previously within the framework of the sp. gr. P3 are discussed.