The Generation of HCl in the System NaC1-KC1-HzO-Quartz at 600°C: Implications Regarding HCl in Natural Systems at Lower Temperatures

In experiments at 600°C in the system NaCI-KCI-H20, within the analytical uncertainty, stoichiometric quantities of C1 and total alkali metals (Na+K) appear to dissolve in steam coexisting with chloride-rich brine at high pressures in the absence of solid salt. In contrast, at lower pressures, where steam coexists with precipitated salts, significant excess chloride as associated hydrogen chloride (HCIO) dissolves in steam. The HCI" appears to be generated by the reaction of solid NaCl(s) (halite) with steam, producing solid NaOH(s) that diffuses into halite, forming a solid solution. Where HCI" is present highly associated NaOH" as well as associated NaCI" appear to dissolve in steam, and the solubility of each is increased as the mole fraction of NaOH(s) in halite increases. In our quasi-static experiments, compared to dynamic flow-through experiments of others, higher initial ratios of H20/NaCI have resulted in higher mole fractions of NaOH(s) in solid solution in halite and, accordingly, higher solubilities of NaCI" and NaOH" dissolved in steam. Addition of quartz to the system NaCI-KCI-H20 results in the formation of sodium disilicate by reaction of silica with NaOH(s) and an order of magnitude increase in the concentration of HClO dissolved in steam. In natural hydrothermal systems at lower temperatures where brine or brine plus steam are present in the absence of precipitated salt, the pH of the brine is controlled mainly by base exchange reactions involving a variety of silicates that fix Na+/H+ and K+/H+ activity ratios. Where feldspars are present pH values generally are near neutral. Where mica, but no feldspar is present pH values may become only moderately acid. High acidity in salt-absent brine systems occurs only where all feldspars and mica have been altered to other minerals (generally pyrophyllite/ kaolinite or alunite). The situation changes significantly when salt precipitates. Hydrolysis produces HCI" by the reaction of water with NaCl when halite is present. The NaOH(s) that is produced as a byproduct is likely to react with quartz plus various alumino-silicates, producing a variety of alteration products and allowing steam to become greatly enriched in HClO compared to the composition of steam that is attained in the simple system NaCI-KCI-H20 with halite present. Also, when a natural high-temperature hydrothermal system changes from one in which the pore fluid is brine to one in which the pore fluid is dry steam there is a drastic change in Na+/H+ and K+/H+ activity ratios in the pore fluid because the hydrogen ions that were predominantly dissociated species in the brine become predominantly associated species in steam. The net result is the stabilization of alkali feldspars in contact with steam that may contain appreciable HCI" that is produced by the reaction of precipitated salt with the steam.