Modelling regularity of Si/Al distributions in disordered tectosilicates - application to zeolites with the GIS framework type.

As an extension of a previous work, we analyse ordered and disordered Si/Al distributions in a few tectosilicates. The method is based on an analysis of the inter-relations between maximal independent sets in a labelled quotient graph of the net. The analysis suggests the presence of specific disordered substructures, called here alveoli, coexisting with fully ordered parts, defining partial order in submicroscopic domains. The method is first illustrated with bikitaite, chabazite and analcime and fully developed for natural zeolites with the GIS framework type. We show that the principle of maximal independence applied to the labelled quotient graph of the net gis can be used to justify composition and order/disorder of the two ordered phases gismondine and amicite and of the disordered phases garronite and gobbinsite. To this list of natural zeolites, we add the disordered, synthetic Na-P2 phase. The results are in good agreement with published 29Si magic-angle spinning NMR data for bikitaite, chabazite and analcime.