Transmission Electron Microscopy Analysis and First-Principles Calculations of Phase Transition from Coesite to Quartz

The research on the phase transition from coesite to quartz has great significance to understand the pressure and temperature tracks of ultra-high pressure (UHP) metamorphic rocks during the exhumation process. In this work, the coesite relics occurred as inclusions in UHP eclogites from Shima, Dabie Mountains, China, were investigated by transmission electron microscopy (TEM) analysis. The TEM results showed that the coesites were partly transformed and surrounded by a rim of polycrystalline quartz with a palisade structure. Furthermore, it could be obviously observed that coesite exhibited a very low dislocation density, while retrograde quartz with a high dislocation density. The corresponding selected area electron diffraction (SAED) pattern showed that the coesite and quartz coexisted in the coesite inclusions. The quartz was formed by the back-transformation of coesite. The cell parameters of coesite and quartz were geometrically optimized and the phase transition pressure of SiO2 from coesite to quartz was carried out according to the first-principles calculations based on the density functional theory (DFT). It was found that the coexistence pressure of coesite and quartz was about 4.1 GPa according to the simulative plot of enthalpies versus pressure.