Clinoptilolite-heulandite polymorphism: structural features from computer simulation

Static lattice computer modelling techniques are applied to investigate the framework and extra-framework cation distribution in clinoptilolite/heulan dite zeolitic materials. The results emphasise the close coupling between t he framework Al distribution and the location of the extra-framework cation s. In the majority of the polymorphs, Al is predicted to be preferentially localised at the T2 and T3 crystallographic sites. Extra-framework cation d istributions are often controlled by a strong driving force for the locatio n of Ca2+ close to Al-rich regions. Cation-cation repulsion also play a sub stantial role in controlling the position of extra-framework cations. We fi nd significant differences in both the distribution of Al in the framework and in the location and distribution of the extra-framework cations dependi ng on the exact composition of the heulandite-structured material considere d.