New Data on the Isomorphism in Eudialyte-Group Minerals. IX: Blocky Isomorphism at Key Sites. Crystal Structure of Fe-Deficient Eudialyte from the Khibiny Massif

The crystal structure and crystallochemical features of Fe-deficient manganese-rich eudialyte from the Khibiny massif (Kola Peninsula) have been investigated by X-ray diffraction analysis, electron probe microanalysis, and IR spectroscopy. The trigonal unit-cell parameters of the mineral are found to be a = 14.199(1) A, c = 30.305(1) A, and V = 5291.2(2) angstrom(3); sp. gr. R3m. The crystal structure is refined to the final reliability factor R = 4.46% in the anisotropic approximation of atomic displacements, using 1937 reflections with F > 3 sigma(F). The idealized crystallochemical formula of the mineral (Z = 3, Roman numerals indicate the coordination numbers (CNs) of cations in the key sites) is N1-5(Na,H3O)(15)(V-X) (M1)(Ca,Mn)(6)(VI) M-2(Fe-IV,Na-VI,Na-V,Mn-VII,Mn-V)(3) M-3(Si-IV,Nb-VI)Si-M4(IVZ)(Zr,Ti) [Si24O72] (OH,Cl,SO4)(3) (H2O,OH) center dot 1.5H(2)O. The regularities of block isomorphism in eudialyte-group minerals are discussed.