Pressure‐Induced Coordination Changes in a Pyrolitic Silicate Melt From Ab Initio Molecular Dynamics Simulations

Abstract

With ab initio molecular dynamics simulations on a Na-, Ca-, Fe-, Mg-, and Al-bearing silicate melt of pyrolite composition, we examine the detailed changes in elemental coordination as a function of pressure and temperature. We consider the average coordination as well as the proportion and distribution of coordination environments at pressures and temperatures encompassing the conditions at which molten silicates may exist in present-day Earth and those of the Early Earth's magma ocean. At ambient pressure and 2,000 K, we find that the average coordination of cations with respect to oxygen is 4.0 for Si-O, 4.0 for Al-O, 3.7 for Fe-O, 4.6 for Mg-O, 5.9 for Na-O, and 6.2 for Ca-O. Although the coordination for iron with respect to oxygen may be underestimated, the coordination number for all other cations are consistent with experiments. By 15 GPa (2,000 K), the average coordination for Si-O remains at 4.0 but increases to 4.1 for Al-O, 4.2 for Fe-O, 4.9 for Mg-O, 8.0 for Na-O, and 6.8 for Ca-O. The coordination environment for Na-O remains approximately constant up to core-mantle boundary conditions (135 GPa and 4000 K) but increases to about 6 for Si-O, 6.5 for Al-O, 6.5 for Fe-O, 8 for Mg-O, and 9.5 for Ca-O. We discuss our results in the context of the metal-silicate partitioning behavior of siderophile elements and the viscosity changes of silicate melts at upper mantle conditions. Our results have implications for melt properties, such as viscosity, transport coefficients, thermal conductivities, and electrical conductivities, and will help interpret experimental results on silicate glasses. Key Points: • The proportion and distribution of cation-to-oxygen coordination environments in a pyrolite melt have been determined as a function of depth • We examine the distribution of lifetimes of cation-oxygen species at select pressure-temperature conditions • Our results are compared to previous ab initio calculations on mafic silicate melts and to experiments on various silicate phases Supporting Information:• Supporting Information S1• Data Set S1Correspondence to:

The Critical Point and the Supercritical State of Alkali Feldspars: Implications for the Behavior of the Crust During Impacts

“…In terms of actual species rather than average coordination numbers, the two feldspar systems behave in a similar way to other silicate melts (Karki et al, 2018;Solomatova & Caracas, 2019). Figures 5 and 6 show the proportion of the SiO x and AlO x species in the NaAlSi 3 O 8 and KAlSi 3 O 8 systems, respectively, for two relevant temperatures below (3000 K) and above (6000 K) the critical temperature as a function of density.…”

Buoyancy and Structure of Volatile‐Rich Silicate Melts

“…In any case, the choice of using a third‐order or fourth‐order Birch‐Murnaghan EOS has only a negligible effect on the relative densities (Figure S2). Between 0 and about 40 GPa, pyrolite melt undergoes rapid densification as the coordination numbers of the cations by oxygen atoms sharply increase (Solomatova & Caracas, 2019), meaning that the density increases more rapidly at upper‐mantle pressures and more gradually with increasing pressure. Pure pyrolite melt is less dense than the solid mantle, modeled with the preliminary reference Earth model (Dziewonski & Anderson, 1981), by about 0.5–1.0 g/cm 3 , depending on the temperature of the melt (Figure 1).…”

“…Consequently, the compression is accommodated in the melts by more efficient packing, which is reflected in the increase of the coordination number. In a detailed analysis of the structure of dry, non‐carbonated, pyrolytic melts (Solomatova & Caracas, 2019) we have seen that all the coordination polyhedra of the cations by anions increase consistently with compression. At ambient pressure, the melts are dominated by long‐living SiO 4 (and AlO 4 ) tetrahedra, with minor 3‐fold and 5‐fold coordination polyhedra.…”

“…The reference melt has the pyrolite composition, which is an approximant to the bulk silicate Earth composition (McDonough & Sun, 1995), and which already served in previous studies (Caracas et al., 2019; Solomatova & Caracas, 2019; Solomatova et al., 2019). It is a six‐component melt with a molar composition of 0.5Na 2 O–2CaO–1.5Al 2 O 3 –4FeO–30MgO–24SiO 2 .…”

Redox-structure dependence of molten iron oxides

“…Hence more sophisticated theoretical work and/or ab initio simulations are necessary to elucidate the precise ratios of 4-, 5-and 6-fold coordinated Fe 2+ -O and Fe 3+ -O in liquid FeO x , particularly in order to understand transport properties, and to address chargetransfer and small-polaron conductance phenomena. In fact, during the writing of this article we note that two ab initio molecular dynamics simulations on FeO x -bearing silicate melts have been published 32,33 . It is anticipated that our experimental pair distribution functions for molten iron oxides presented herein will provide ambient-pressure benchmarks to test models relevant to a wide range of industrial and natural processes.…”