Liquid-Vapor Phase Relations in the Si-O System: A Calorically-Constrained van der Waals-Type Model

1James A. D. Connolly
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005059]
1Earth Sciences Department, Swiss Federal Institute of Technology, Zurich, Switzerland
Published by arrangement with John Wiley & Sons

This work explores the use of several van der Waals (vW) type equations of state (EoS) for predicting vaporous phase relations and speciation in the Si-O system, with emphasis on the azeotropic boiling curve of SiO2-rich liquid. Comparison with the observed Rb and Hg boiling curves demonstrates that prediction accuracy is improved if the a-parameter of the EoS, which characterizes vW forces, is constrained by ambient pressure heat capacities. All EoS considered accurately reproduce metal boiling curve trajectories, but absent knowledge of the true critical compressibility factor, critical temperatures remain uncertain by ~500 K. The EoS plausibly represent the termination of the azeotropic boiling curve of silica-rich liquid by a critical point across which the dominant Si oxidation state changes abruptly from the tetravalent state characteristic of the liquid to the divalent state characteristic of the vapor. The azeotropic composition diverges from silica toward metal-rich compositions with increasing temperature. Consequently, silica boiling is divariant and atmospheric loss after a giant impact would enrich residual silicate liquids in reduced silicon. Two major sources of uncertainty in the boiling curve prediction are: the heat capacity of silica liquid, which may decay during depolymerization from the near-Dulong Petit Limit heat capacity of the ionic liquid to values characteristic of the molecular liquid; and the unknown liquid affinity of silicon monoxide. Extremal scenarios for these uncertainties yield critical temperatures and compositions of 5200-6200 K and Si1.1O2-Si1.4O2. The lowest critical temperatures are marginally consistent with shock experiments and are therefore considered to be more realistic.


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