Modular structure of a sodium-rich analogue of eudialyte with the doubled c-parameter and the R3 symmetry

The structure of a new sodium-rich representative of the eudialyte group wi th the ideal formula (Na,Sr,K)(35)Ca12Fe3Zr6TiSi51O144(O,OH,H2O)(9)Cl-3 was established by the X-ray diffraction analysis (R = 0.054 based on 3503 \F \). The unit-cell parameters are a = 14.239(1), c = 60.733(7) Angstrom, V = 10663.9 Angstrom (3), sp. gr. R3. The mineral structure is characterized b y in-layer order of cations resulting in doubling of the c-parameter and th e formation of two modules, the composition and the structure of one of whi ch corresponds to eudialyte (with an impurity of kentbrooksite), the protot ype of the second module is alluaivite, Lowering of the symmetry is caused by ordering of Ca atoms and the Ca-replacing elements in the alluaivite mod ule and by the displacements of the alkali cations from the m plane. (C) 20 01 MAIK "Nauka/Interperiodica".