English: Schematic illustration of microbial reduction of ferric iron in an oxide crystal. The microorganisms exist within the aqueous phase and possibly attached to the surface of the oxide mineral. They can transfer the electrons to solid-phase ferric iron using a variety of strategies, as described in Chapter 5. Regardless of the strategy, the electrons must pass through the interface between the solid and aqueous phases. The size of that interface varies with the surface area of the oxide crystal. Thus, oxide surface area can influence the rate of the reaction.