Iron reduction in nontronite-type clay minerals: Modelling a complex system

Reduction–oxidation or redox processes constitute a class of important reactions in a wide range of mineral environments. The specific focus in this investigation is on iron-bearing (ferruginous) clay minerals, where the redox reaction has important consequences for their structural and chemical integrity. Although this process has been studied experimentally, it is not yet fully understood where and how this occurs within clay mineral layers. The investigation presented here addresses this question from first principles using density functional theory (DFT), planewaves, pseudopotentials and periodic cells. The first issue addressed is that of simulating a dynamic reduction process using static models. Careful consideration is paid to the introduction of artificial electrostatic interactions, their subsequent identification and the effect these may have on the results. As a consequence of these considerations, three sets of models based on nontronite (Fe2(Si,Al)4O10(OH)2) are presented. The electronic structures of these clay mineral models are allowed to relax, to attain their own state of redox. By extensively analysing the Mulliken charges, magnetic states and orbital occupancy of iron, aluminium, silicon and oxygen, we have been able to draw firm conclusions about the relative reduction of iron within the tetrahedral and octahedral sheets of three varieties of nontronite. Reduction occurs to the greatest extent in the octahedral sheet iron and oxidation in the tetrahedral sheet iron. As these results reflect general, local environments within a clay mineral, they are therefore applicable to similar local environments and thus provide the foundations for further studies into more complex, geochemical systems.