Flexibility and distortion of the framework of natrolite: crystal structures of ion-exchanged natrolites
Baur, Werner H.; Kassner, Dethard; Kim, Choon-Hee; Sieber, Norbert H. W.
European Journal of Mineralogy Volume 2 Number 6 (1990), p. 761 - 770
published: Dec 19, 1990
manuscript accepted: Jun 18, 1990
manuscript received: Apr 2, 1990
ArtNo. ESP147050206003, Price: 29.00 €
Abstract The crystal structure of the Li-exchanged form of natrolite, Li1.6Na0.4Al2Si3O10 • 2H20, was refined based on 568 Fobs, R = 0.071. For comparison the structure of a natrolite, Na2Al2Si3O10 • 2H2O, was also refined (1278 Fobs, R = 0.040). The diffraction data collected for K-natrolite were of insufficient quality, therefore a computer simulation of the K-form was based on the experimentally determined cell edges and space group. The mean rotation angles ψ of the chains (composed of 4 = 1 secondary building units) relative to the a and b cell axes are observed to be 25° (Li-compound), 24° (Na-compound) and 18° (K-compound). A detailed comparison shows that only the transformation to K-natrolite conforms approximately to the currently widely accepted model of rotating chains. Surprisingly the Li-form instead achieves the observed shortened cell constants a and b by a distortion of the chain itself, the rotation is only a secondary effect. Both cation exchanged forms display also shortened c-axes, which is an indication that the chains of 4 = 1 units are tilted and twisted within themselves. It is most likely that the angle of the hinge at the oxygen atom O2 is in Na-natrolite already at the lower attainable limit beyond which it would become energetically unfavorable, and thus it cannot be reduced any further when the Na-natrolite is transformed by ion-exchange into a Li-natrolite.