Redox Properties of Structural Fe in Smectite Clay Minerals

Redox reactions of structural Fe in clay minerals play important roles in biogeochemical processes and for the fate of contaminants in the environment. Many of the redox properties of Fe in clay minerals are, however, poorly understood, thus limiting the knowledge of the factors that make structural Fe participate in electron transfer reactions. This chapter summarizes the current state of knowledge on the redox properties of structural Fe in clay minerals. In the first part, we review the various spectroscopic observations associated with structural Fe reduction and oxidation and how changes in Fe oxidation state affect the clay mineral structure and the binding environment of Fe in the octahedral sheet of planar 2:1 clay minerals. In the second part, we show how information on the structural alterations and arrangement of Fe can be interpreted to assess the apparent reactivity and the thermodynamic redox properties of structural Fe in clay minerals.