We have conducted a series of experiments in 0.01-molal chloride solutions at 200°C and 2 kbar on the muscovite–quartz–sanidine equilibrium for variable solid/fluid ratios, which in these experiments are proportional to the surface area of the solids. The quench pH decreases with increasing solid/fluid ratios for runs with starting solution compositions in the sanidine field (i.e., relatively alkaline solutions), but increases with increasing solid/fluid ratios for runs with starting solutions in the muscovite field (i.e., relatively acid). The two trends intersect at a solid/fluid ratio of ~1/16, which is the ratio that yields the narrowest equilibrium reversals; in turn these reversals agree well with the independently-calculated log K (200°C 2 kbar).
For the same reaction in 0.01-molal chloride solutions at 205°C and 17 bar vapor pressure, the same trend of quench pH-vs.-solid/fluid ratio is observed for the runs approaching equilibrium from the muscovite field as for the 2 kbar runs, but no clear trend emerges from the runs approaching equilibrium from the sanidine field. Taken together, the experiments at 2 kbar and 17 bar indicate that surface reactions cannot account for the two trends in quench pH; if they did, the trends observed on approaching equilibrium from both sides would be the same, which they are not. We conclude that dilute solutions are appropriate for collecting high-temperature/high-pressure equilibrium data provided one uses the rapid-quench technique with solid/fluid mass ratios of ~1 /16.
The rapid-quench, dilute chloride solution technique was also used to determine log K vs. T for the muscovite–quartz–sanidine reaction at 2 kbar and solid/fluid ~1/16 over the interval 200–500°C. The log K's, determined via aqueous-speciation calculations for each T and P, coincide with the log K's calculated independently from the thermodynamic properties of the reactants and products.