Stability Fields of Iron Minerals in Anaerobic Marine Sediments

The pH of anaerobic marine sediments is in general constant relative to $$pS^{--} (-log as--)$$. Consequently, the two-dimensional representation of the stability fields of authigenic iron minerals for anaerobic marine sediments can be expressed in the useful form of $$Eh-pS^{--}$$ diagrams (at constant pH and $$P_{CO_{2}}$$), calculated from thermodynamic data. Such diagrams have been constructed for the minerals pyrite, pyrrhotite, siderite, magnetite, and hematite. Some equilibrium relations suggested by the diagrams have been confirmed by comparison with natural measurements of Eh and $$pS^{--}$$ and mineralogical determinations. Considerably more observations are needed to check the possible importance of metastability upon the applicability of the diagrams when used to interpret the mineralogy of ancient sedimentary rocks.