The thermodynamics of water in quartz

Abstract

The thermodynamic model of Doukhan and Trepied (1985) for water solubility in quartz, based on the (4H)Si substitution, has been developed to take into account the variation in oxygen fugacity under different buffering conditions, as well as the solubility of the quartz in the water. An evaluation of the thermodynamic parameters has then been made using analogous data on grossular and hydrogrossular. This evaluation, although approximate, leads to predicted solubilities that are lower than some published experimental values but are consistent with other observations indicating that the solubility is relatively low, less than 100H/106 Si at most, even at the highest temperatures and pressures. The predictions give a monotonically increasing solubility with increasing temperature at given pressure, and an increase in solubility with increasing pressure at given temperature up to a maximum, beyond which the solubility decreases with further increase in pressure. The variation of solubility with oxygen fugacity at fixed pressure and temperature broadly mirrors the observations of Ord and Hobbs (1986) although not predicting the finer details. An interstitial H2O model is also considered and shown to be probably less important as a mechanism for water solubility than the (4H)Si mechanism. Hydrogen/alkali exchange depends on hydrogen fugacity rather than water fugacity and is predicted to be more important at lower oxygen fugacities.