The compression of framework minerals: beyond rigid polyhedra

The concept of rigid polyhedral units has been very successful in explaining a broad range of physical, chemical and thermodynamic properties of framework minerals at high temperatures. In this contribution experimental data on the high-pressure behaviour of selected framework minerals is reviewed and the ability of the rigid unit model to predict the observed compressional mechanisms is assessed. We find that the majority of framework minerals compress by approximately following a rigid-unit mode but that mode is slightly modified by the small amounts of compression of the framework polyhedra. In perovskites, the direction along the rigid-mode coordinate is determined by whether the framework octahedra are more or less compressible than the extra-framework cation site. We conclude that the concept of relatively rigid polyhedra following rigid-unit modes during compression remains "a useful fiction" (Hazen, 1988) for initiating the analysis of the compressional behaviour of complex framework structures.