Abstract: The method which use X ray diffraction to measure the size of atom and ion and determine crystal structure is most important step for mankind get into microscopic view world. At usual temperature and pressure, the connection pattern and the arrangement rule of various atom and ion have already elaborated in the crystal chemistry in detail. In this paper, the study on the crystal chemistry characteristics of ionic compounds and minerals was emphasized under high pressure. Under high pressure when the pressure increases, the cations of silicon in oxide or silicate minerals occurred migration from low coordination number polyhedra to high coordination number polyhedra. This migration is due to the change with radius ratio of cations to anions. Under high pressure the ionic radii of oxygen and silicon have been calculated for different SiO₂ polymorphs. It is indicated that the ionic radius of oxygen is decreasing gradually with the increase of pressure, but the cation radius of silicon is increasing with the increase of pressure and the occurrence of phase transitions. This kind of phenomenon may be due to the transition from ionic compounds to metallic phases under high pressure.

Key words: polymorphs of SiO_2, , high-pressure phase transition crystal chemistry, coordination polyhedron, ionic radius