SITE OCCUPANCY IN NONTRONITE STUDIED BY ACID DISSOLUTION AND MOSSBAUER-SPECTROSCOPY

The dissolution of two Ca2+-exchanged nontronite samples has been studied in 10% HCl. Early acid-dissolution studies (Osthaus, 1954) have indicated that after two hours of dissolution most of the octahedral Fe3+ ((VI)Fe3+) would be removed leaving mainly tetrahedral Fe3+ ((IV)Fe3+) in the nontronite structure. In the present study, Fe-57 Mossbauer spectra of acid-treated samples were recorded and fitted with two octahedral Fe3+ (2 x (VI)Fe3+) and two octahedral and one tetrahedral (2 x (VI)Fe3+, 1 x (IV)Fe3+) doublet models. The Mossbauer spectra of acid-treated Garfield nontronite samples could be adequately fitted with two-doublet models but acid-treated Hohen Hagen nontronite samples could not. Isomer shift and quadrupole splitting values obtained from the two-doublet models corresponded to (VI)Fe3+ and not (IV)Fe3+, as was suggested by the Osthaus (1954) experiment. When an (IV)Fe3+ doublet was included in the model used to fit the Mossbauer spectra of acid-treated Garfield nontronite samples, a slight increase in the intensity of the (IV)Fe3+ doublet occurred with increasing dissolution, but this was much lower than indicated by Osthaus (1954). No trend in the intensity of the (IV)Fe3+ doublet was observed for acid-treated Hohen Hagen nontronite. Therefore, acid treatment appears to remove (VI)Fe3+ and (IV)Fe3+ from the nontronite structure at about the same rate. Mossbauer spectroscopy, infrared spectroscopy and X-ray powder diffraction data indicate that the nontronite that remains undissolved following acid treatment is structurally similar to the untreated nontronite.