Crystal chemistry of the high hydrous analogue of gismondine-Sr and the role of water in the extra-framework cations ordering

Abstract The crystal structure of the highly hydrous analogue of the K-rich gismondine-Sr from Bellerberg paleovolcano, Eifel Mountains, Germany, was studied by single-crystal X-ray diffraction. Electron microprobe analyses (energy-dispersive spectroscopy analyses) provided the empirical formula Sr 1.86 Ca 1.13 Ba 0.02 K 1.47 Na 0.48 (Al 7.79 Fe 0.13 Si 8.08 O 32.025 )·14.97H 2 O. The mineral is orthorhombic, space group P 2 1 2 1 2, a = 14.1790(2), b = 10.6062(2), c = 13.8989(2) Å, V = 2090.19(6) Å 3 and Z = 2. The crystal structure of the mineral ( R = 0.0374 for 4963 unique reflections with I > 2σ( I )) is characterized by the gismondine-type (GIS) framework with ordered Al and Si atoms in tetrahedral sites. Partially ordered extra-framework cations and water molecules fill the cages of the framework. The higher hydration degree increases the ordering of extraframework cations in the structure. The higher hydration degree increases the ordering of extraframework cations in the structure. The phenomenon is discussed in ion-exchanged GIS zeolites as well as natural zeolites of PHI-type structures.