Advances in research on high-pressure phase transitions of (Mg,Fe)SiO3 pyroxene

(Mg,Fe)SiO3 is one of the fundamental components of the solid Earth. It primarily occurred as pyroxene minerals (orthopyroxene and clinopyroxene) in the crust and upper mantle. To investigate the phase transition behaviors of (Mg,Fe)SiO3 (pyroxenes) under conditions of high temperature and high pressures is of great significance for understanding the material compositions and physicochemical states of the Earth′s interior, and the evolution of the deeply subducted slabs. This paper mainly reviews advances in the studies on the high-pressure phase transitions of orthopyroxene and clinopyroxene ((Mg,Fe)SiO3) under relatively low-temperature conditions ( corresponding to the environment of a cold subducting slab). It focuses on the characteristics of crystal structures and high-pressure transition mechanisms of these two kinds of pyroxenes (categorized as Type-I phase transition characterized by the change of geometric forms of the SiO4 tetrahedral chains and Type-II phase transition characterized by the increase of cation coordination numbers). Based on this, we conducted a detailed analysis of the complex effects of element substitutions (such as Fe, Al, Ca, Ni, etc.) and trace water on the phase transition pathways and phase transition pressures, and then also systematically reviewed various kinds of possible polymorphs of (Mg,Fe)SiO3 that may existed in the Earth′s interior and their stability fields. Finally, we pointed out that more efforts should be made to conduct experimental measurements and geophysical explorations of the elastic properties of metastable (Mg,Fe)SiO3 phases, and to carry out researches on the phase transition kinetics of these metastable (Mg,Fe)SiO3 phases, in order to reveal the time scale and spatial distribution of them existed in the deep Earth.