Abstract--The reduction and reoxidation of three nontronite samples, GAN (API H-33a, Garfield, Washington), SWa-1 (ferruginous Washington smectite), and NG-1 (Hohen Hagen, Federal Republic of Germany) were studied with visible absorption and M6ssbauer spectroscopy. The intensity of the intervalence electron transfer (IT) band at 730 nm in these nontronites was monitored during reduction and reoxidation at 277, 294, and 348 K. The results showed that the intensity of the band followed the number of Fe(II)-O-Fe(III) groups in the clay crystal, increasing to a maximum at about Fe(II):total Fe = 0.4; upon complete reduction, the band decreased to about the intensity of the unaltered, oxidized sample. With reoxidation of the sample with O2, the intensity of the band increased sharply, followed by a gradual decay back to the original, oxidized intensity. The ultimate level of Fe reduction achieved was at least 92%. Concomitantly, the color changed from yellow through green, blue-green, dark blue, light blue, and light gray as the Fe(II) content increased. The GAN nontronite was more difficult to reduce than the SWa-1 or NG-1 samples. The rate and level of reduction increased with the amount of reducing agent added.

Abstract--The effect of Fe oxidation state on the specific surface area, Sin, ofnontronite was studied using the < 2-tzm, Na § fraction of the SWa-1 and Garfield nontronite reference clays. The reduction of structural Fe 3+ in the octahedral sheet of the nontronite decreased Sm as measured by the adsorption of 2-ethoxyethanol (ethylene glycol-monoethyl ether, EGME). The swellability in water of the nontronite also decreased during reduction. The amount of nonexchangeable Na +, on the other hand, increased with increasing Fe 2+ content and was highly correlated with EGME adsorption (r = -.985). The relationship between Sm and Fe 2+ was attributed to the collapse of partially or fully expanded layers to unexpanded layers.

Abstract--The effects of chemical reduction of structural Fe 3 § in nontronite SWa-1 (ferruginous smectite) on intervalence electron transfer (IT) and magnetic exchange were investigated. Visible absorption spectra in the region 800-400 nm of a chemical reduction series of the SWa-1 nontronite revealed an IT band near 730 nm (13,700 cm-'). Both the intensity and position of this band were affected by the extent of Fe reduction. The intensity increased until the Fe 2+ content approached 40% of the total Fe, then decreased slightly with more Fe 2+. The position of the band also shifted to lower energy as the extent of reduction increased.Variable-temperature magnetic susceptibility measurements showed that the magnetic exchange in unaltered nontronite is frustrated antiferromagnetic, but ferromagnetic in reduced samples. Magnetic ordering temperatures are in the range 10-50 K, depending on the extent of reduction. The ferromagnetic component in the magnetization curve increased with increasing Fe 2 § in the crystal structure. The positive paramagnetic interaction likely is due to electron charge transfer from Fe 2 § to Fe 3 § through such structural linkages as Fe2+-O-Fe 3 § (perhaps following a double exchange mechanism), which is consistent with the visible absorption spectra.

Abstract--The effects of reduction and reoxidation of octahedral Fe 3+ on the exchange of structural hydrogen in nontronite were determined using tritium (all) as a label element. The uptake of H from the surrounding solution of nontronite suspensions increased as the reduction of structural Fe 3+ 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 H20 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.

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