Infrared and Mössbauer Studies of Reduced Nontronites

Abstract

Abstract--lnfrared and M6ssbauer spectroscopy show that the extent of the reduction of nontronite is dependent on the chemical composition of the nontronite and on the nature of the reducing agent. Hydrazine reversibly reduces about 10% of the iron in all of the nontronltes studied irrespective of composition and it is suggested that the resulting ferrous iron occurs only in distorted octahedral sites. Similar conclusions are reached for the dithionite reduction of the nontronites containing little tetrahedral iron, but for those with more than one in eight silicons replaced by iron, changes brought about by dithionite treatment are irreversible due to dissolution of appreciable quantities of iron. Results from both spectroscopic techniques suggest that iron in tetrahedral sites is preferentially dissolved and that up to 80% of the structural iron can be reduced.Evidence is presented for the formation in these extensively reduced nontronltes of a small amount of a mica-like phase resembling celadonite or glauconite, and, as dithionlte is used for the pretreatment of soils, the implication of this observation is briefly discussed.The use of deuterated hydrazine as a reducing agent has enabled the nontronite absorption band near 850 cm -1 to be assigned to a Si-O (apical) stretching vibration, which is inactive in the infrared for perfect hexagonal symmetry, but which is activated by distortions in the tetrahedral layer.

Synthesis of 1:1 And 2:1 Iron Phyllosilicates and Characterization of Their Iron State by Mössbauer Spectroscopy

“…Iron phyllosilicates or iron-containing clay minerals have been studied for their oxidation-reduction reactions Heller-Kallai, 1976a, 1976b;Russell et al, 1979;Stucki et al, 1984aStucki et al, , 1984bWu et al, 1989), their catalytic and adsorption properties (Fanale and Cannon, 1979;Heller-Kallai et al, 1984;Wang and Huang, 1989), and their magnetic properties (Coey et aL, 1982;Gangas et al, 1985). Because iron has two common oxidation states (ferrous and ferric), and a ferric ion shows two common coordination numbers (fourfold and sixfold), the crystallization of iron silicates and iron-containing phyllosilicates is complex and difficult to control (Roy and Roy, 1954;Harder, 1973Harder, , 1977Harder, , 1989Petit and Decarreau, 1990).…”

Effects of Reduction and Reoxidation of Structural Iron on the Surface Charge and Dissolution of Dioctahedral Smectites

“…These ions could block some cationexchange sites and render the CEC an inaccurate measure of the layer charge. Clays containing tetrahedral Fe are particularly susceptible to irreversible alteration when reduced with dithionite (Russell et al, 1979), but even clays with very little tetrahederal Fe, such as the Garfield nontronite, can suffer some dissolution (Stucki et al, 1984). It appears, however, that the dissolution of clays containing little or no tetrahedral Fe is minimal if pH is maintained near neutral, as evidenced by the reversibility of the CEC in the Garfield nontronite.…”

Mössbauer and Infrared Study of Heat-Treated Nontronite

“…In the absorption of the 782-cm -t band, a contribution from the Si-O vibrations is also possible (Russell et al, 1979). Three peaks occur also in region (3); the peaks at 495 and 430 cm -~ were attributed to the vibrations of Si-O-Fe, and the third at 455 cm -1 to the Si-O-Si bending vibrations (Goodman et al, 1976).…”

“…Mtssbauer spectroscopy may detect their presence, but because of the nanosize dimensions of the grains (Dickson and Cardile, 1986), the oxides are usually supermagnetic at 80 K whereas their presence as magnetic sextes in the spectra is obvious at 4.2 K. Treatment of nontronite according to Mehra and Jackson (1960) removes all partially crystalline iron-oxide species. Sodium dithionite used in this treatment causes partial reduction of structural Fe 3+ to Fe 2 § However, reoxidation by air is easily obtained by air-drying and thus the process is reversible (Russell et al, 1979;Komadel et al, 1990). All spectra discussed are free of iron oxides based on 4.2-K M6ssbauer spectra.…”