57Fe Mössbauer Spectroscopy of Montmorillonites: A New Interpretation

C. M. Cardile and J. H. Johnston
Chemistry Department, Victoria University of Wellington, Private Bag, Wellington, New Zealand

Abstract: The 57Fe Mössbauer spectra of several montmorillonites, measured at room temperature and 453 K, showed a considerably broadened Fe3+ resonance which can be computer-fitted with a similarly broadened Fe3+ doublet. In some spectra, particularly if all linewidths were constrained to be equal, this broadened Fe3+ resonance was further resolved into overlapping inner and outer Fe3+ doublets, also having broad linewidths. In accordance with recent electron diffraction evidence, the assignment by previous workers of the inner doublet to Fe3+ in the octahedral sites having the cis-arrangement of OH groups and the outer doublet to the octahedral site having the trans-arrangement of OH groups is incorrect. Instead, the Fe3+ was found to be located largely in the trans-octahedral sites. Because of the relatively low iron content of the montmorillonite examined, the next and more distant neighboring-cation environment varied considerably about the octahedral Fe3+ ions. This variation produced a broadened experimental resonance, and the resulting two-doublet computer fits probably represent the mean extremes of a continuum of slightly different Fe3+ resonances arising from the variable nature of the environment surrounding these such trans-sites, rather than distinct cis- and trans-sites. In addition, a small resonance indicating the substitution of Fe3+ into the tetrahedral sites was observed. The interlayer species probably influenced the Mössbauer resonance of Fe3+ in the tetrahedral and octahedral sites.

Key Words: Iron • Montmorillonite • Mössbauer spectroscopy • Octahedral sites • Tetrahedral sites

Clays and Clay Minerals; June 1986 v. 34; no. 3; p. 307-313; DOI: 10.1346/CCMN.1986.0340311
© 1986, The Clay Minerals Society
Clay Minerals Society (www.clays.org)