TABLE OF CONTENTS With very few exceptions the numerous polymorphs of SiO 2 all consist of SiO 4 tetrahedra linked through their corners, thus forming three-dimensional framework structures. The topology of the tetrahedral linkage and the efficiency of space filling are different for the polymorphs. For a given type of framework, for example, that of quartz or cristobalite, space filling can be improved by so-called displacive transformations from a more open high-temperature form (e.g., "high", "h", or " ?-quartz") to a denser form stable at lower temperatures ("low" or " ?-quartz"). These transformations do not change the topology of the framework, i.e., chemical bonds in a crystal can be deformed, but are not broken and rearranged.
Many alumino-silicates of the general formula MAlSi xO 2 x +2, M being a univalent ion or one half of a bivalent ion, also have framework structures. There, AlO 4 tetrahedra are linked up with SiO 4 tetrahedra in an ordered or disordered way, with the M ions occupying cavities in the framework and providing charge neutrality. Some alumino-silicates adopt the frameworks of SiO 2 polymorphs; M.J. Buerger coined the term "stuffed derivatives" for them.
The high-quartz and keatite-type alumino-silicates are such stuffed derivatives. Both types of framework are relatively dense, compared with, for example, those of tridymite and cristobalite. Therefore, only small univalent or divalent M ions can be accommodated...
