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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.