Crystallochemical model and prediction for zeolite-type structures

Abstract

A model for zeolite-type structures, which considers anion coordinations, is established on the basis of crystallochemical rules. Careful observation of 42 structures of zeolite-type minerals verifies the model. The numerical analysis which follows takes account of a scheme for charge transfer in the structures and allows structural chemical conditions to be established for these structures using simple numerical expressions. Accordingly, the atom coordinations in structures can be calculated directly from the chemical composition, which means that prediction of the atom coordinations in zeolite-type structures is now possible. In this paper, this prediction is carried out for some 30 known zeolite-type structures whose cavities, beside water oxygens or other anions, are filled with one type of cavity cation. The results are compared with the observations. It is shown that even the coordination of the cavity cation can be predicted within a fairly good approximation. For zeolite types with an interrupted framework, breaking of the tetrahedral framework is shown to be a consequence of charge equilibrium between the tetrahedral framework and the anions in cavities. A global structural analysis of zeolite-type structures has attracted greater interest as more and more synthetic or natural zeolite-type structures have become known. Refinements of several of these structures are, however, often incomplete because of disorder. At this stage of knowledge, the model and following numerical formulations may find several applications: i.e. in evaluating structures from chemical formulae, for the determination of possible order in disordered structures, for the synthesis of new materials, and for classification. This is a first attempt to define whole structures from chemical compositions using numerical expressions.