Abstract

Zeolites are microporous aluminosilicate minerals. A zeolite can be simply modelled as a set of vertex-pinned rigid tetrahedra: typically a silicon atom and four surrounding Oxygen atoms form a relatively stiff tetrahedron, with each Oxygen atom common to two tetrahedra. The Oxygen acts as a spherical joint, as the Si-O-Si bond angle is relatively flexible. The tetrahedra can be connected together in many different ways to give zeolites with various voids and channels, and this structure is essential to their important uses as molecular sieves and catalysts.

The interesting structural characteristic of zeolite frameworks is the equality between the number of rigid-body freedoms of the tetrahedra, and the number of constraints at the hinges. In fact this implies that there must be at least three inextensional deformation modes for the framework, and the presence of symmetry may increase this number. This insight may have relevance to understanding why there are a relatively small number of actual zeolites (around 200), compared with the several million hypothetical structures that have been found.