The effects of reduction and reoxidation of octahedral Fe3+ on the exchange of structural hydrogen in nontronite were determined using tritium (3H) as a label element. The uptake of H from the surrounding solution of nontronite suspensions increased as the reduction of structural Fe3+ increased. Similarly, the loss of H from the structure increased as the reduction increased. The results are generally consistent with a reduction mechanism involving the loss of structural OH, leaving the affected Fe sites with less than six-fold coordination. The attenuation of increased negative charge on the clay layer, however, was less than predicted by such a mechanism.

During the reoxidation of reduced nontronite in suspension, about one-third of the H remaining as part of the structure following reduction was lost, whereas twice that amount of H was incorporated into the structure from the surrounding solution. A reoxidation mechanism is proposed whereby H2O from the surrounding solution is incorporated into the mineral structure followed by the elimination of a hydrogen ion, returning the Fe to six-fold coordination. This mechanism implies the reversibility of Fe reduction in nontronite.

The 57Fe Mössbauer spectra of a series of untreated and Ca-saturated nontronites showed a predominant Fe3+ resonance which was computer-fitted with two Fe3+ doublets defining iron in non-equivalent cis-FeO4(OH)2 octahedral sites. In most spectra a doublet indicating tetrahedral Fe3+ was fitted and in one untreated sample a doublet indicating interlayer Fe3+ was identified. In a further untreated sample the interlayer iron was present as Fe2+. Upon Ca-saturation the interlayer iron was displaced. It also appears that the interlayer iron was present in at least two different interlayer sites. From the computer-fitted data it was clear that the interlayer cations have a significant effect on the Mössbauer resonances of iron in the two non-equivalent cis-octahedral and the tetrahedral sites of nontronite.