Structure of Ion-exchanged Forms of Chabazite

Abstract

THE structure proposed by Wyart¹ for chabazite includes only the case for the divalent calcium ions, in the positions ± (0.17, 0.17, 0.17). In the case of a chabazite containing monovalent ions, these must also be placed on the triad axis if the symmetry D_3d⁵ is to be maintained without the formation of a ‘defect structure’. Barrer² has shown that ion-exchanged forms of chabazite rich in the alkali metal cations, among others, give X-ray powder photographs showing negligible alterations in spacings when compared with the parent mineral. This indicates that the alumino-silicate network is essentially the same for all the cations, from lithium to cæsium. Initially, it would appear suitable to place these ions at the points (0, 0, 0), (0.5, 0.5, 0.5) and ± (0.25, 0.25, 0.25). However, these four positions are not all equivalent to one another, and, further, the situation of a cation at (0.5, 0.5, 0.5), for example, where it is in the centre of a cage which is otherwise devoid of atoms, is unlikely.