Hydration state and activity of aqueous silica in H2O-CO2 fluids at high pressure and temperature

Quartz solubility in H 2 O-CO 2 fluids was measured at 800 °C and 10 kbar. Mixed fluids were generated from hydrous oxalic acid, silver oxalate, silver carbonate, and liquid H 2 O; solubility was determined by weight changes of the quartz crystals. Stringent blank tests and weighing procedures were used to establish the CO 2 and H 2 O contents of experimental fluids. Using experimentally constrained models of H 2 O activity and mixing of silica monomers and dimers, a logarithmic plot of monomer activity vs. H 2 O activity yields a linear data array of constant slope n insignificantly different from 4 (n = 4.024, R 2 = 0.997), where n is the total monomer hydration number. Moreover, all high-quality quartz solubility data in H 2 O-CO 2 fluids at lower temperature and pressure fall on the same line when SiO 2 and H 2 O activities are calculated with this formulation. The same analysis for the dimer yields slope n of 7 (n = 7.049, R 2 = 0.996). Our results show that neutral silica monomers and dimers have fixed stoichiometry of Si(OH) 4 ·2H 2 O and Si 2 O(OH) 6 ·4H 2 O (i.e., two solvating H 2 O molecules per Si) in H 2 O-CO 2 fluids for a large range of crustal metasomatic processes. This provides a simple basis for modeling H 2 O-CO 2 -SiO 2 fluids over a range of pressure, temperature, CO 2 , and SiO 2 activity.