Silicate pyroxenes, amphiboles, and micas are important components in the Earth’s crust, upper mantle, and in meteorites (Ringwood 1975, Anderson 1989). A detailed knowledge of crystal structures and phase-transition mechanisms in these minerals is, therefore, of great geophysical importance in understanding planetary interiors. The first crystal structure investigation of a pyroxene was reported by Warren and Bragg (1928) who solved the monoclinic C 2/ c diopside structure. Warren and Modell (1930) solved the structure of orthorhombic Pbca hypersthene, and the monoclinic P 21/ c pyroxene was predicted by Ito (1950), who postulated that Pbca orthopyroxene was a “space group twin” of clinopyroxene. Subsequently, Morimoto et al. (1960) published the first descriptions of P 21/ c pyroxene (pigeonite and clinoenstatite) structures. A synthetic orthorhombic pyroxene with Pbcn symmetry was first described by Smith (1959) and called protoenstatite. While working on the pyroxenes, Warren also solved the structures of C 2/ m tremolite and Pnma anthophyllite (Warren 1929, Warren and Modell 1930). Subsequent work on amphiboles showed similar trends to those in the pyroxenes with the determination of the structure of P 21/ m cummingtonite (Papike et al. 1969) and Pnmn protoamphibole (Gibbs et al. 1969). Jackson and West (1930) determined the structure of muscovite; the different polymorphs were described by Hendricks and Jefferson (1939), Heinrich et al. (1953), and Smith and Yoder (1956). In the years since these pioneering studies were completed, there have been many investigations of silicate chain and sheet structures and we now have a rather complete understanding of their crystal chemistry. More recently, the new frontier has been to explore how their structural chemistry changes with temperature and pressure and to better understand their physical and chemical characteristics under the conditions that exist within the …