POSITIONS OF CATIONS AND MOLECULES IN ZEOLITES WITH THE MORDENITE-TYPE FRAMEWORK .8. DEHYDRATED SODIUM-EXCHANGED MORDENITE

Natural mordenite was ion-exchanged in NaCl solution and dehydrated at 300°C [a 17.92(1) b 20.31(1) c 7.480(7)Å at room temperature]. Dehydration reduced the symmetry from Cmcm to Pbcn. All diffractions violating the C-centering are diffuse, and in one crystal are split into doublets indicating a domain structure with a (true cell) ∼5a (pseudo-cell). The diffuse diffractions disappear upon rehydration. Diffraction intensities were collected using a wide scan for diffuse diffractions. Site occupancies are: I', 3.1Na; IV, 2.6Na; VI, 1.5Na. Electron microprobe analysis yielded Na7.3K0.2Ca0.03Al8.3Si39.9O96, and some cationic Al, or OH-, or both, may be needed for charge balance. (Na(I') is displaced 0.53Å from its ideal position at (0,0.5,0), perhaps because of electrostatic repulsion between adjacent ions. An ordered model with 3.2Na and 0.8 vacancies in site I' provides a qualitative explanation of the domain structure. Na(IV) is displaced laterally from the center of an eight-ring. The effect of decationization" on site occupancy and molecular sorption is discussed. © 1979."