Ab Initio Two-Phase Molecular Dynamics on the Melting Curve of SiO2

Ab initio two-phase molecular dynamics simulations were performed on silica at pressures of 20-160 GPa and temperatures of 2 500-6 000 K to examine its solid-liquid phase boundary. Results indicate a melting temperature (T-m) of 5 900 K at 135 GPa. This is 1 100 K higher than the temperature considered for the core-mantle boundary (CMB) of about 3 800 K. The calculated melting temperature is fairly consistent with classical MD (molecular dynamics) simulations. For liquid silica, the O-O coordination number is found to be 12 along the T-m and is almost unchanged with increasing pressure. The self-diffusion coefficients of O and Si atoms are determined to be 1.3x10(-9)-3.3x10(-9) m(2)/s, and the viscosity is 0.02-0.03 Pas along the T-m. We find that these transport properties depend less on pressure in the wide range up of more than 135 GPa. The eutectic temperatures in the MgO-SiO2 systems were evaluated and found to be 700 K higher than the CMB temperature, though they would decrease considerably in more realistic mantle compositions.