Dissymmetrization in Eudialyte-Group Minerals. VIII: Refinement of the Ikranite Crystal Structure in the Framework of the P3 Model

The crystal structure of ikranite, low-calcium and high-oxonium eudialyte-group member, was repeatedly studied by the X-ray diffraction method within the space group Р3. The trigonal unit-cell parameters are a = 14.17521(8) Å, c = 30.1673(2) Å, V = 5249.60(6) Å3. The crystal structure of ikranite with the idealized formula (Na,H3O)15(Ca,Mn,REE)6Fe ₂^3 + Zr3( ,Zr)( ,Si)Si24O66(O,OH)6Cl·nH2O, n = 2–3, was studied previously using a limited set of diffraction data within the sp. gr. R3m. Full experiment with refining the structural model in the sp. gr. Р3 (158 unique positions, R = 0.046, 4912 unique reflections with F > 3σ(F)) revealed some additional structural features of this mineral, in particular, the existence of a six-membered ring of edge-sharing CaO6 and NaO6 octahedra, along with a ring from alternating CaO6 and MnO6 octahedra, established previously in some eudialyte-group minerals of the oneillite type.