First-Principles Molecular Dynamics Study of the Structure of MgSiO₃ Melt at High Temperatures and High Pressures

The microstructures of MgSiO₃ melt and their variation with temperature and pressure were investigated based on first-principles molecular dynamic simulations at high pressures (0-144 GPa) and high temperatures (2 000-6 000 K).The calculated first peak positions of the pair correlation function of O—Si, O—Mg and O—O under the condition of 0 GPa and 2 000 K are 0.163 5, 0.1 970 and 0.269 5 nm, respectively, which are consistent with the previous experimental values.As the pressure and temperature change, the structure of MgSiO₃ melt undergoes a significant change.Especially when the pressure increases, the structure becomes denser.When the temperature is below 5 000 K, the average bond lengths between two atoms decrease with the increasing temperature with density 4.59 g/cm³.While under nomal or higher pressure, the average bond length change with the increasing temperature is not obvious.At 133 GPa and 4 000 K, the average bond lengths of O—Si, O—Mg and O—O are 0.161 0, 0.183 5 and 0.230 0 nm, respectively; the average Si—O coordination number increases from 4 to 6, and the number of bridging oxygen ratio increases from 31.3% to 72.9%, from atmospheric pressure to the core-mantle boundary.The knowledge of MgSiO₃ melt microstructure is important to understand the mantle silicate fluid nature of mantle dynamics.