Molecular dynamics simulations of pressure and temperature effects on MgSiO3 and Mg2SiO4 melts and glasses

Abstract

Abstract. Ab-initio interionic potentials for Mg 2+, Si 4 +, and 0 2. have been used in molecular dynamics (MD) simulations to investigate diffusivity changes, pressureinduced structural transitions, and temperature effects on polymerization in MgSiO 3 and Mg2SiO 4 melts and glasses. The potential gives reasonable agreement with the 0.1 MPa radial distribution function of MgSiO3 glass. Maxima in the diffusion coefficients of Si 4+ and 0 2-occur as pressure is increased on the MgSiO3 melt. The controlling structural mechanism for this behavior is the Q 1 species of SiO4 tetrahedra. Mg 2 + diffusion coefficients decrease monotonically with pressure in both melt compositions. Increasing Mg 2 § coordination number and population of 3-and 4-membered SiO4 rings with pressure combine to hinder translation of the Mg 2 § ions. The dominant changes in structure with pressure are a decrease in the intertetrahedral (Si-O--Si) angle up to approximately 4 g/cm 3 and coordination changes of the ions above this density. Temperature effects on viscosity in these simulated melts are indirectly studied by analyzing polymerization changes with temperature. Polymerization and coordination numbers increase with decreasing temperature and a small quench rate effect is observed. Fair agreement is found between the MD simulations and experimental equation of state for Mg2SiO4, but the equation of state predictions for MgSiO3 melts are much less accurate. The zero pressure volume, Vo, is significantly higher and Ko is lower in the simulations than empirical values. The inadequacies reflect error in using the ionic approximation for polymerized systems and a need to collect more data for a variety of molecular configurations in the development of ab-initio potentials.

In situ diffraction studies of magnesium silicate liquids

“…Although binary silicate systems are relatively simple and only crudely approximate natural silicate liquids, the diffraction data illustrate that the increased distortion of the local environment around magnesium ions can be correlated with the departure from Arrhenius law viscosity. We anticipate that this diffraction data will provide a rigorous test of molecular dynamics simulations on MgO-SiO 2 melts, which have so far shown poor agreement with the glass pair distribution function [44,45]. For liquids with 50% SiO 2 , corresponding to the composition of the mineral enstatite, the liquid comprises a sparse network of SiO 4 tetrahedra with highly mobile magnesium ions although the silicate network controls the liquid rheology.…”

Thermodynamic properties of MgSiO3 perovskite derived from large scale molecular dynamics simulations

“…Given that we work at constant volume, this implies that the classically calculated pressure will be a slight underestimate of the corresponding quantum mechanical pressure. However, the quantum correction to the pressure calculation will usually be much smaller than the classical fluctuations in the pressure (Kubicki and Lasaga 1991). Similarly, the quantum corrections to the actual structure will also be negligibly small (Matsui 1989).…”

Theoretical study of a five-coordinated silica polymorph

“…4,5,[13][14][15][16] Although all of the observed stable and metastable crystalline phases of silica consist of either IV Si or VI Si, there has been great interest in the possibility of forming phases in which silicon has fivefold coordination by oxygen ( V Si). This species has been predicted in theoretical studies of the effects of pressure and temperature on silicate melts and glasses [17][18][19][20][21][22][23][24][25][26][27] and inferred experimentally by 29 Si NMR and vibrational spectroscopy in alkali silicate liquids and glasses. 16,[28][29][30][31][32][33][34][35][36] Silicon has also been found in fivefold coordination with oxygen in a number of organosilicon compounds.…”

“…Simulations with these potentials have predicted that the formation of V Si is promoted by high pressures; for SiO 2 glass these studies show that IV Si species are gradually replaced by V Si and VI Si with increasing pressure. 18,19,27,40 Such simulations permit the exploration over a broad range of the effects of pressure, temperature, and deviatoric ͑differential͒ stresses on phase stability and physical properties. In fact, recent molecular-dynamics simulations on ␣-quartz under nonhydrostatic stress found evidence for a crystalline phase of SiO 2 composed entirely of SiO 5 groups.…”