Tiettaite K4Na12Fe3+Si16O41(OH)4 ⋅ 2H2O: A Mineral with a Novel Type of Microporous Heteropolyhedral Framework

Microporous silicate tiettaite from the Khibiny alkaline massif (Kola Peninsula, Russia) has been studied using X-ray diffraction analysis, electron probe microanalysis, and Raman spectroscopy of single crystals and powders. Its chemical formula is refined to be K4Na12Fe3+Si16O41(OH)4 ⋅ 2H2O. The Raman spectrum of tiettaite contains lines at 3650, 3450, 1097, 1066, 980, 920, 681, 635, 569, 499, 458, 404, 272, 144 cm–1. The orthorhombic unit-cell parameters are a = 28.866(7) A, b = 10.707(3) A, c = 17.425(7) A, V = 5385(3) A3, Z = 4, sp. gr. Cmcm. The tiettaite crystal structure has been solved for the first time from single crystal and refined by the Rietveld method to the final Rwp = 2.47% and RBragg = 1.38%. The structure based on a new-type microporous heteropolyhedral framework formed by one-dimensional modules [Si28Na16Fe4K4O110]76– elongated along the c axis. Each module consists of complex ribbons with compositions of [Si14O39]22– and [Fe2K2Na6O32]50– compose a framework via shared vertices of half-occupied silicon–oxygen tetrahedra Si5 and Si6. Tiettaite structure contains a three-dimensional system of crossed channels with effective channel widths of 2.61 × 7.50, 2.46 × 2.92, and 3.60 × 3.90 A2, running along the [100], [010], and [001] directions, respectively. The presence of a system of broad channels allows one to consider tiettaite as promising ion exchanger. Regarding the information complexity parameters, tiettaite can be assigned to minerals with very complex structure.