- © 2000 Mineralogical Society of America
Framework structures, which feature three-dimensional networks of relatively rigid polyhedral units that share corners with one another, encompass a wide range of natural and synthetic compounds of importance in the Earth sciences, solid state chemistry, condensed matter physics, and materials sciences. Many frameworks consist entirely of corner-sharing tetrahedra such as SiO4, AlO4, BeO4 and PO4. Examples include phenakite, major crustal-forming groups of minerals such as the feldspars, feldspathoids, most of the silica polymorphs, as well as technologically-important groups of compounds such as zeolites. Other frameworks incorporate network-forming tetrahedra and octahedra and include the scandium and zirconium tungstates and molybdates, which have ScO6 and ZrO6 octahedra corner-linked to WO4 and MoO4 tetrahedra, respectively, and high-pressure framework silicates with alternating groups of 4- and 6-coordinated silicon (e.g. Hazen et al. 1996). Examples of the latter group include MgSiO3 with the garnet structure, Ca2Si2O5 with the titanite structure and K2Si4O9 with the wadeite structure. Perovskites are examples of frameworks composed entirely of corner-linked octahedra, with no tetrahedral elements.
Many insights into the high-pressure, high-temperature behavior of framework structures derive from the fact that frameworks are composed of relatively rigid polyhedral units and the forces that act within these units are much stronger than the forces that act between them. Hazen and Finger (1982) developed the polyhedral approach to describe changes of crystal structures under high pressure, high temperature and with variable composition. They characterized the compression or thermal expansion by means of the behavior of the polyhedral components of the structure. They found, for example, that the bulk modulus of a phase largely depends on the bulk moduli of individual polyhedra which build up the structure and the manner in which …