Liquid‐Vapor Coexistence and Critical Point of Mg₂SiO₄ From Ab Initio Simulations

Abstract

Hypervelocity impact‐driven vaporization is characteristic of late‐stage planet formation. Yet the behavior and properties of liquid‐vapor mixtures of planetary materials of interest are typically unknown. Multiphase equations of state used in hydrodynamic simulations of planet impacts therefore lack reliable data for this important phenomenon. Here, we present the first constraints on the liquid‐vapor critical point and coexistence phase boundary of Mg2SiO4 computed from ab initio molecular dynamics simulations. We found that the vapor is depleted in magnesium and enriched in silica and oxygen, while the coexisting liquid is enriched in magnesium and depleted in oxygen, from which we infer vaporization is incongruent. The critical point was estimated from an equation of state fit to the data. The results are in line with recent calculations of MgSiO3 and together confirm that extant multiphase equation of state (EOS) models used in planetary accretion modeling significantly underestimate the amount of supercritical material postimpact.

Temperature and Density on the Forsterite Liquid‐Vapor Phase Boundary

“…This forsterite ANEOS model (version SLVTv1.0G1, Stewart et al., 2019) provides a good fit to the shock Hugoniot, liquid‐vapor curve, and isentropes connecting the Hugoniot and vapor curve up to several 100s GPa. This model’s critical point also agrees well with ab initio calculations of the forsterite critical point (Townsend et al., 2020) at ρ c = 0.52 ± 0.03 g/cm 3 , T c = 6,240 ± 200 K, and P c = 0.13 ± 0.02 GPa.…”

“…Release temperature experiments on silica combined with first principles molecular dynamics simulations noted that vapor thicknesses less than 200 μm have negligible absorption at visible wavelengths (Kraus et al., 2012). For Mg 2 SiO 4 , the vapor phase includes Mg and MgO (Townsend et al., 2020) complicating the absorption spectra of the liquid‐vapor mixture. Absorption by the liquid‐vapor mixture changes with time because of the creation of more vapor as the rarefaction waves propagate up range, thickening the mixed phase layer.…”

“…The high pressure measurements on the principal Hugoniot (Davies et al, 2020;Root et al, 2018), preliminary analyses of the experiments presented in this work, and constraints on the critical point, now published in Townsend et al (2020), were used to constrain a revised forsterite ANEOS-based model (labeled New ANEOS). The ANEOS code package was updated to include a user adjustable heat capacity in the liquid and was focused on fitting high pressure Hugoniot data and the liquid and vapor regions of the phase diagram, see Stewart et al (2020).…”

The critical point and the supercritical regime of MgO

“…Our simulations with 216 atoms yield almost the same result, in terms of energy, pressure, and equation of states, as the simulations with 512 atoms. This is consistent with the analysis of finite-size effects in forsterite (Mg 2 SiO 4 ) melts [26], where a system size of 56 atoms already approaches the critical temperature within the accuracy range presented in this research. Furthermore, employing the van der Waals correction for the dispersive forces at the smallest density investigated here caused no appreciable difference for bond lifetimes or pressure.…”

“…But these studies stop short at relatively low temperature, and they fail to reach the CP and to explore the supercritical regime. For the large majority of rock-forming minerals, such as feldspars [24], MgSiO 3 , and Mg 2 SiO 4 , the vaporization is incongruent [21,[24][25][26].…”