A computer simulation study of natural silicate melts. Part II: High pressure properties

Abstract

The thermodynamic, structural and transport properties of natural silicate melts under pressure are investigated by molecular dynamics simulation with the help of a force field recently introduced by us [Guillot B. and Sator N. (2007) A computer simulation study of natural silicate melts. Part I: low pressure properties. Geochim. Cosmochim. Acta 71, 1249-1265]. It is shown that the simulation reproduces accurately the bulk moduli of a large variety of silicate liquids as evaluated from ultrasonic studies. The equations of state (EOS) of the simulated melts are in good agreement with the density data on mid-ocean ridge basalt, komatiite, peridotite and fayalite as obtained either by sink/float experiments or by shock-wave compression. From the structural point of view it is shown that the population of [5] Al and [6] Al species increases rapidly upon initial compression (0-50 kbar) whereas for Si these highly coordinated species are found in a significant abundance (>5%) only above $50 kbar for [5] Si and $100-150 kbar for [6] Si. This increase of the coordination of network formers is not the only response of the melt structure to the densification: there is also a large redistribution of the T-O-T (T = Si, Al) bond angles with the pressure and noticeably upon initial compression in rhyolitic and basaltic liquids. Furthermore, a detailed analysis of the population of bridging oxygens (BO) and nonbridging oxygens (NBO) points out that the polymerization of the melt generally increases when the pressure increases, the magnitude of this polymerization enhancement being all the more important that the melt is depolymerized at low pressure. The role of triclusters (threefold coordinated oxygens to network former cations) is particularly emphasized in acidic and basaltic liquids. The pressure-induced redistribution of the oxygen atoms through the melt structure is also stressed. Finally, the simulation predicts a nonmonotonic behavior of the diffusivity of network former ions when the pressure increases, a feature with depends on the melt composition. This could have a counterpart in the electrical conductivity at sufficiently high temperature when the viscosity of the liquid is low.

Thin film lubrication of hexadecane confined by iron and iron oxide surfaces: A crucial role of surface structure

“…The COMPASS force-field was used for lubricant, 26 while the Buckingham potential, as presented in Table II, was applied for the oxide surfaces. 27,28 This potential had reproduced the thermodynamic and structural properties of FeO and Fe 2 O 3 in natural silicate melt at both low and high pressure. 27,28 The employed atomic charges for iron oxides, as presented in Table II, are a little higher than those obtained from our previous study using density functional theory (DFT) calculation.…”

Including many-body effects in models for ionic liquids

“…Apart from the intrinsic interest for such systems, they have been studied by computer simulations in many contexts. For example, molten oxides are the main components of magmatic melts [3][4][5][6] , molten fluorides and chlorides are investigated for their importance in many metallurgical processes [7][8][9] and for their potential use in the nuclear industry 10,11 , and room temperature ionic liquids for their use in electrochemical storage applications [12][13][14] or their solvation properties [15][16][17] .…”

Simplicity in melt densification in multicomponent magmatic reservoirs in Earth’s interior revealed by multinuclear magnetic resonance

“…and hence, natural silicate melts in the earth's interior (22). Most of the previous studies focused on the pressure-induced bonding transition in simple model melts (e.g., from single component, to ternary) (e.g., refs.…”