Site Occupancy in Nontronite Studied by Acid Dissolution and Mössbauer Spectroscopy

Vittorio Luca1 and Dugald J. MacLachlan2
Chemistry Department, Victoria University of Wellington P.O. Box 600, Wellington, New Zealand
1Present address: Research School of Chemistry, The Australian National University, Canberra, ACT 2601, Australia.
2Present address: Department of Biology, University College, London WC1E6BT, England.

Abstract: 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+ (VIFe3+) would be removed leaving mainly tetrahedral Fe3+ (IVFe3+) in the nontronite structure. In the present study, 57Fe Mössbauer spectra of acid-treated samples were recorded and fitted with two octahedral Fe3+ (2 × VIFe3+) and two octahedral and one tetrahedral (2 × VIFe3+, 1 × IVFe3+) doublet models. The Mössbauer 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 VIFe3+ and not IVFe3+, as was suggested by the Osthaus (1954) experiment. When an IVFe3+ doublet was included in the model used to fit the Mössbauer spectra of acid-treated Garfield nontronite samples, a slight increase in the intensity of the IVFe3+ doublet occurred with increasing dissolution, but this was much lower than indicated by Osthaus (1954). No trend in the intensity of the IVFe3+ doublet was observed for acid-treated Hohen Hagen nontronite. Therefore, acid treatment appears to remove VIFe3+ and IVFe3+ from the nontronite structure at about the same rate. Mössbauer 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.

Key Words: Acid dissolution • Mössbauer spectroscopy • Nontronite • Tetrahedral iron

Clays and Clay Minerals; February 1992 v. 40; no. 1; p. 1-7; DOI: 10.1346/CCMN.1992.0400101
© 1992, The Clay Minerals Society
Clay Minerals Society (www.clays.org)