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A self-consistent optimization of multicomponent solution properties: ab initio molecular dynamic simulations and the MgO-SiO_2 miscibility gap under pressure

Harvey, Jean-Phillipe and Gheribi, Aïmen E. and Asimow, Paul D. (2015) A self-consistent optimization of multicomponent solution properties: ab initio molecular dynamic simulations and the MgO-SiO_2 miscibility gap under pressure. Geochimica et Cosmochimica Acta, 161 . pp. 146-165. ISSN 0016-7037.

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We propose a new approach to parameterizing the Gibbs energy of a multicomponent solution as a function of temperature, pressure and composition. It uses the quasichemical model in the second nearest neighbour approximation and considers both a polynomial representation (for low pressure) and an exponential decay representation (for moderate-to-high pressure) of the excess molar volume v^(xs) to extend thermodynamic behaviour to elevated pressure. This approach differs from previous configuration-independent regular or associated solution-type models of multicomponent silicate liquids at elevated pressure and can account for any structural or short-range order data that may be available. A simultaneous least squares fit of the molar volume and the molar enthalpy of mixing data obtained from First Principles Molecular Dynamics (FPMD) simulations at various pressures enables complete parameterization of the excess thermodynamic properties of the solution. Together with consistently optimized properties of coexisting solids, this enables prediction of pressure-temperature-composition phase diagrams associated with melting. Although the method is extensible to natural multicomponent systems, we apply the procedure as a first test case to the important planetary model system MgO-SiO_2 using FPMD data found in the literature. One key result of this optimization, which depends only on the derived excess properties of the liquid phase, is that the consolute temperature of the SiO_2-rich miscibility gap is predicted to decrease with increasing pressure. This appears to be in disagreement with available experimental constraints and suggests possible thermodynamic inconsistency between FPMD data and experimental phase equilibrium data in the 0-5 GPa pressure range. We propose a new thermodynamic consistency criterion relating the signs of v^(xs) and other excess properties and discuss the need for precise calculations of derivatives of excess properties. Finally, the potential reappearance of the miscibility gap in the MgO-SiO_2 system above 5 GPa is discussed in light of this work.

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Asimow, Paul D.0000-0001-6025-8925
Additional Information:© 2015 Elsevier B.V. Received 15 August 2014, Accepted 2 April 2015, Available online 13 April 2015. We would like to thank Dr. James Sangster and Dr. Pierre Hudon for their constructive criticisms of the present work as well as the editors and reviewers at GCA for pushing us to improve the manuscript. This work was supported in part by Defence Research and Development Canada and by the US NSF through award EAR-1426526.
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Defence Research and Development CanadaUNSPECIFIED
Record Number:CaltechAUTHORS:20150427-105640207
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Official Citation:Jean-Philippe Harvey, Aïmen E. Gheribi, Paul D. Asimow, A self-consistent optimization of multicomponent solution properties: Ab initio molecular dynamic simulations and the MgO–SiO2 miscibility gap under pressure, Geochimica et Cosmochimica Acta, Volume 161, 15 July 2015, Pages 146-165, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:57004
Deposited By: Ruth Sustaita
Deposited On:27 Apr 2015 19:20
Last Modified:03 Oct 2019 08:19

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