Experimental study of the influence of water on the buffer equilibrium of magnetite–wüstite and wüstite–metallic iron

In experimental studies, water decomposition under reducing conditions results in the high fugacity of hydrogen, which is able to form interstitial solid solutions with wüsite. New experiments demonstrate that the hydrogen fugacities attained in the presence of high water pressure and at the oxygen fugacities imposed by the buffer equilibria magnetite–wüstite (MW) and wüstite-iron (WI) are sufficiently high for the occurrence of hydrogen–oxygen interactions and for the formation of H 2 -saturated compound. Instead of buffer equilibria, hydrogen fugacity was directly controlled in the experiments at 700–1200°C and 200 MPa using the improved Shaw membrane technique in the form of a specially designed cell with Ar–H 2 mixtures; H 2 mole fraction in the mixtures ranged from 0.0 to 0.8. According to the phase rule for systems with perfectly mobile independent components, buffer reactions in the presence of water-bearing phases change into divariant equilibria. The X-ray study of phase composition along Mag–Wus divariant field shows an increase of wüstite content relative to magnetite with increase of hydrogen mole fraction in a fluid. Mass-spectrometric study showed that the relative bulk solubility of hydrogen in buffer phases at 950°C decreases with an increase of iron content in wüstite owing to the change in wüstite stoichiometry under more reducing conditions. Experimental data indicate that the hydrogen solubility in wüstite results in a shift of the magnetite stability field toward reducing region for at 700°C and for at 1200°C, whereas the stability of wüstite with metallic iron practically coincides with anhydrous equilibrium, which is related to the close values of hydrogen solubility in wüstite and in the metallic iron at temperatures below 950°C. However, the hydrogen solubility in metallic iron significantly increases with increasing temperature and its divariant field is shifted to a reducing region.