An inelastic incoherent neutron scattering study of water in small-pored zeolites and other water-bearing minerals

Inelastic neutron scattering spectra of the zeolites natrolite Na2Al2Si3O10 2H2O, wairakite CaAl2Si4O12 2H2O, scolecite CaAl2Si3O10 3H2O, and bikitaite Li2Al2Si4O12. 2H2O, together with the sheet silicate apophyllite KCa4Si8O20(F, OH) 8H2O, are shown here. We show clear trends across these minerals, illustrating the relative influence of hydrogen bonding and cation bonding. For some of these spectra, the bands are clearly separated, permitting a discussion of their assignments. In particular, we can identify librational bands (L) or librational edge (LE) and translational (cation-water stretch) bands (C), and by elimination can tentatively assign bands as hydrogen-bond stretch bands (H), as follows (frequencies given in meV): Natrolite: 13,18 (C) 26 (H), 64,68,87 (L); Apophyllite 17,26 (C), 35 (H) 64,79,98 (L); Scolecite 10 (C), 22,29,36,41 (H), 46 (LE); Wairakite 12 (C), 30 (LE); Bikitaite 13 (C), 22 (H), 34 (LE), 70(L). The results are compared with those for ice and are explained in terms of the nearest-neighbor environment of the water molecule.