A new sampling technique for fluid phases in hydrothermal experiments applied to the determination of the HF-fugacities of the WFQ-buffer

A new sampling device is described which allows the separation of a fluid phase from the solid reactants in hydrothermal experiments at run conditions. The new method has been tested at temperatures up to 1,100° C at a total vapor pressure of 1 kbar.HF-concentrations of the fluid phase in equilibrium with quartz, fluorite, and wollastonite in the reaction CaF₂+SiO₂+H₂O CaSiO₃+2 HF known as the WFQ-buffer (Munoz and Eugster 1969) have been determined by direct chemical analysis using a fluoride electrode. HF-concentrations measured in the fluid phase range from 39±3 ppm at 510° C to 1,968±98 ppm at 820° C. This is equivalent to fugacities of 0.036 bar at 510° C compared to 0.033 as calculated from thermodynamic data, and 2.238 bar at 820° C compared to 2.142 bar. Equilibrium was reversed by starting out from aqueous HF-solutions in which the HF-concentrations were above or below the equilibrium value.