Redox Reactions in Hydrothermal Fluids
Many elements participate in oxidation-reduction reactions in the geothermal/epithermal environment. These include C, S, H, O, N, Fe, Mn, U, W, As, Sb, Bi, Cu, Ag, Au, Te, and Sn. The first six or seven elements listed are much more abundant than the rest and they interact to buffer the redox state; the remaining (and to a large extent the most interesting economically) elements are usually much less abundant, and they only respond to the chemical environment imposed by the dominant redox systems. In this chapter we shall investigate methods of determining the oxidation state of a system, either directly by calculations based on the chemistry of geothermal gases and liquids, or indirectly by interpreting the phases and phases assemblages observed in fossil hydrothermal systems.
Redox reactions are important in such diverse areas as the corrosion and scaling of geothermal production pipes, the interaction of organic matter with fluids, the oxidation of H2s, the precipitation of native metals and pyrite and other sulfides, the destruction of sulfides by oxidation, and the disproportionation of SO2 into H2S and SO4 on cooling from high temperature.