The linear mixing approximation (LMA) is often used in planetary models for calculating the equations of state (EOS) of mixtures. A commonly assumed planetary composition is a mixture of rock and water. Here we assess the accuracy of the LMA for pressure-temperature conditions relevant to the interiors of Uranus and Neptune. We perform molecular dynamics simulations using ab initio simulations and consider pure water, pure silica, and 1:1 and 1:4 silica-water molecular fractions at a temperature of 3000 K and pressures between 30 and 600 GPa. We find that the LMA is valid within a few percent (< ∼5%) between ∼150 and 600 Gpa, where the sign of the difference in inferred density depends on the specific composition of the mixture. We also show that the presence of rocks delays the transition to superionic water by ∼70 GPa for the 1:4 silica-water mixture. Finally, we note that the choice of electronic theory (functionals) affects the EOS and introduces an uncertainty of the order of 10% in density. Our study demonstrates the complexity of phase diagrams in planetary conditions and the need for a better understanding of rock-water mixtures and their effect on the inferred planetary composition.
