Oxidation-reduction mechanism of iron in dioctahedral smectites: II. Crystal chemistry of reduced Garfield nontronite

The crystallochemical structure of reduced Garfield nontronite was studied by X-ray absorption pre-edge and infrared (IR) spectroscopy, powder X-ray d iffraction, polarized extended X-ray absorption fine structure (P-EXAFS) sp ectroscopy, and texture goniometry. Untreated and highly reduced (>99% of t otal Fe as Fe2+) nontronite samples were analyzed to determine the coordina tion number and the crystallographic site occupation of Fe2+, changes in in -plane and out-of-plane layer structure and mid-range order between Fe cent ers, and to monitor the changes in structural and adsorbed OH/H2O groups in the structure of reduced nontronite. Contrary to earlier models predicting the formation of fivefold coordinated Fe in the structure of nontronites u pon reduction, these new results revealed that Fe maintains sixfold coordin ation after complete reduction. In-plane P-EXAFS evidence indicates that so me of the Fe atoms occupy trans-sites in the reduced state, forming small t rioctahedral domains within the structure of reduced nontronite. Migration of Fe from cis-to trans sites during the reduction process was corroborated by simulations of X-ray diffraction patterns which revealed that about 28% of Fe2+ cations exist in trans sites of the reduced nontronite, rather tha n fully cis occupied, as in oxidized nontronite. Out-of-plane P-EXAFS resul ts indicated that the reduction of Fe suppressed basal oxygen corrugation t ypical of dioctahedral smectites, and resulted in a flat basal surface whic h is characteristic of trioctahedral layer silicates. IR spectra of reduced nontronite revealed that the dioctahedral nature of the nontronite was los t and a band near 3623 cm(-1) formed, which is thought to be associated wit h trioctahedral [Fe2+],OH stretching vibrations. On the basis of these resu lts, a structural model for the reduction mechanism of Fe3+ to Fe2+ in Garf ield nontronite is proposed that satisfies all structural data currently av ailable. The migration of reduced Fe ions from cis-octahedra to adjacent tr ans-octahedra is accompanied by a dehydroxylation reaction due to the proto nation of OH groups initially coordinated to Fe. This structural modificati on results in the formation of trioctahedral Fe2+ clusters separated by clu sters of vacancies in which the oxygen ligands residing at the boundary bet ween trioctahedral and vacancy domains are greatly coordination undersatura ted. The charge of these O atoms is compensated by the incorporation of pro tons, and by the displacement of Fe2+ atoms from their ideal octahedral pos ition toward the edges of trioctahedral clusters, thus accounting for the i ncoherency of the Fe-Fe1 and Fe-Fe2 distances. From these results, the idea l structural formula of reduced Garfield nontronite is Na-1.30[Si7.22Al0.78 ] [Fe3.652+Al0.32Mg0.04]O-17.93(OH)(5) in which the increased layer charge due to reduction of Fe3+ to Fe2+ is satisfied by the incorporation of proto ns and interlayer Na.