Thermodynamic analysis of quartz and cristobalite solubilities in water at saturation vapor pressure

The degree of polymerization of silica in aqueous solution is calculated from quartz and cristobalite solubility and the free energy of the polymorphic transition. At temperatures below 130° C, the existing solubility data are not all compatible with a simple monomeric solute model. A combination of monomeric and tetrameric silica species is shown to be compatible with most of the chemical data on silica solutions. The analysis indicates that the low temperature solubilities of quartz and/or cristobalite must be more accurately defined or that the presence of polymeric silica in solution has been overlooked.