Pyrite fixing oxidation state

As a second example, we constrain a fluid s oxidation state by assuming equilibrium with pyrite (FeS2). As before, direct information on this variable can be difficult to obtain, so it is not uncommon for modelers to use mineral equilibrium to fix a fluid s redox state. The choice of pyrite to buffer oxidation state, however, is perilous because pyrite sulfur, which is in the S1- oxidation state, may dissolve by oxidation to sulfate (S6+),... 

The reaction between pyrite and hematite fixes the initial oxidation state to a reducing value (log fo2 = —54). We then set a reaction path in which we add oxygen to the fluid... 

---