Abstract:
A total of 34 solubility experiments using the ''model''-pelite mineral assemblage microcline + low albite + andalusite + quartz were performed at 600°C and 0.2 GPa over a total chloride range of 0.03-2.9 molal. The concentrations of sodium, potassium, aluminum, and silica were measured and the results are compared with four different thermodynamic datasets. The K/Na ratio was approached from below and above for the thermodynamically buffered mineral assemblage microcline + low albite + andalusite + quartz. Tight brackets were obtained for experiments performed in up to 1 molal chloride concentration. From 0.03 to ~1 molal chloride concentration, a constant K/Na ratio of 0.33 was obtained. At 3 molal chloride concentration the K/Na ratio decreases to 0.28. Experiments accidentally running out of quartz show a different trend. The K/Na ratio systematically decreases from 0.39 to 0.27 from low to high chloride concentrations. Silica and aluminum concentrations were only approached from below. The silica concentration of 0.1 molal is constant over the investigated total chloride concentration. Aluminum is more concentrated at low Clt°t, decreasing from 0.001 to 0.0005. Comparison of experimental results with computed aqueous speciation generated with four different thermodynamic databases shows good qualitative agreement. For the most part, predicted total concentrations using the extended Debye-Huckel activity model for charged solutes, and setting the Setchenow coefficient to zero for neutral species, are generally within an order of magnitude of those measured, which is likely within the uncertainties of the calculations.