QUARTZ, ALBITE AND DIOPSIDE SOLUBILITIES IN H2O-NACI AND H2O-CO2FLUIDS AT 0.5-0.9 GPA

Abstract

The solubility of quartz and incongruent dissolution ('apparent solubility') of albite and diopside in H2O-NaCl and H2O-CO2 fluids have been determined at pressures up to 0.9 GPa (9 kbar) and temperatures of 500-900 °C. Solubilities of quartz and albite decrease with increasing salt concentration [X(NaCl)](salt-out effect), whereas the solubility of diopside increases with increasing X(NaCl)(salt-in effect). Quartz solubilities in the systems H2O-NaCl and H2O-CO2 are very similar and are proportional to X(H2O)2. Quartz solubility in NaCl-rich brines does not change with pressure under the P-T conditions of our experiments. At 0.9 GPa and 800 °C, albite solubility in pure water is higher (~100 g/kg H2O) than that of quartz (~74 g/kg H2O), but at NaCl concentrations >6 mol% these solubilities are very similar. Albite dissolution is slightly incongruent; formation and composition of secondary and quench phases (paragonite, Na-margarite, amorphous quench spheres) indicate that the solution is enriched in SiO2 and Na2SiO3. As fluid composition changes from H2O towards more CO2-rich fluids or NaCl-rich brines, the solubilities of albite and quartz decrease by about one order of magnitude, but are still measurable at X(H2O)2O-CO2 fluids at 0.9 GPa indicates that complexing is dominated by SiO2.4H2O for water-rich fluids, but for X(H2O)<0.7 the mean solvation number decreases to ≤3. This speciation is similar to that reported previously for lower pressures, and is in agreement with recent high P-T spectroscopic data for the system H2O-SiO2. For dissolution of quartz in both H2O-CO2 and H2O-NaCl fluids, the molecular proportions of silica to water are almost the same at any X(H2O). Assuming similar non-ideal water-salt interactions irrespective of whether the water is in the fluid or is complexed with silica, then the speciation of silica appears to be similar in both H2O-CO2 and H2O-NaCl fluids under the experimental conditions. We conclude that the speciation of silica in both H2O-CO2 and H2O-NaCl fluids at 0.9 GPa and 800 °C is comparable, is dominated by Si(OH)4·2H2O and/or hydrated species with lower hydration numbers, and is also comparable to that proposed for lower P-T conditions.