Thermodynamics of Element Volatility and its Application to Planetary Processes Paolo A. Sossi Institut de Physique du Globe de Paris, 1 rue Jussieu, F-75005, Paris, France
[email protected] Bruce Fegley, Jr. Planetary Chemistry Laboratory, Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, Missouri, USA
[email protected] This is the accepted, pre-proof version of the paper: Sossi PA & Fegley B (2018) Thermodynamics of Element Volatility and its Application to Planetary Processes. Rev Mineral Geochem 84 (1):393–459. doi: 10.2138/rmg.2018.84.11 1.0. Introduction Despite its importance in geological sciences, our understanding of interactions between gas and condensed phases (comprising solids and liquids) remains clouded by the fact that, often, only indirect evidence remains for their occurrence. This arises from the tendency for the vapour phase to escape from the condensed phase with which it interacts, owing to its much lower density and thus greater volume. For a gas that is sufficiently tenuous that interactions do not occur between its constituent molecules, this relationship is quantified in the ideal gas law (Clapeyron 1834): 푃푉 = 푛푅푇 (1) where 푃 is the total pressure exerted by the gas, 푉 its volume, 푛 is the number of moles, 푅 the gas constant (8.3145 Jmol-1K-1, Horstmann, 1873) and 푇 the absolute temperature. One mole of an ideal gas at 273.15 K and 105 Pa (standard temperature and pressure for gases)1 has a molar volume of 22,711 cm3/mol, 103 × greater than typical silicate liquids or minerals.