STABILITY OF SPATIAL DISTRIBUTION OF WATER MOLECULES IN NATURAL ZEOLITES. Nikolay K. Moroz, Institute of Inorganic Chemistry, Siberian Branch Russian Academy of Sciences, 630090 Novosibirsk, Russia

Proton NMR study of great variety of natural heulandites and clinoptilolites reveals that the water environment in these isomorphic aluminosilicates remains virtually unchanged under variations of the Al/Si ratio and composition of extra cations. In the case of cation-exchanged forms of these crystals the situation is exactly the opposite: as a rule, ion-exchange leads to a drastic rearrangement of water molecules in the framework cavities. Certain distinctions between the water environments in hydrothermal crystals and in zeolitic tuffs have been found.

All NMR measurements and spectra analysis were performed on a portable automatic NMR analyser specially designed for proton NMR studies of hydrogen-containing minerals. The water environments have been characterised via the spatial distributions of proton-proton (p-p) vectors which are determined by the populations of structural positions of water in a crystal and by orientational disordering of molecules in each position. As measures of this distribution, the averaged over lattice values of squares of p-p vector direction cosines were used. These values are easily obtained from NMR line shape analysis when fast diffusion of H₂O molecules by direct interchange of sites takes place [1,2].

The most likely reason of stability of the spatial distribution of water molecules in differ heulandites and clinoptilolites is the H₂O(H₂O dipolar interaction. The energy of this interaction can be minimized in natural zeolites at stage of mineral formation by appropriate arrangement of extra cations, the Al and Si atoms, and possible by some deformations of framework. The energy minima are not achieved in artificial cation-exchanged forms, because in this case ion-exchange is performed in already formed framework.

References:

1. Ducros P. Bull. Soc. Fr. Mineral. Cristalogr. 83, 85 (1960).

2. Gabuda S.P. and Lundin A.G. Soviet Physics - JETP 55, 1066 (1968).