Mössbauer spectra of nontronites: Structural implications and characterization of associated iron oxides

Mössbauer spectra of three nontronite samples (Garfield H33a and SWa‐1 from Washington State, U.S.A., and a sample from Hundsangen, Germany), taken between room temperature and 4.2 K in the velocity range ± 10.5 mm/s, showed between 2.9 and 13% of the total iron to be bound in goethite and not in the nontronite structure. This extraneous iron limits the utility of room temperature spectra for the assignment of structural iron sites in these samples. An adequately good distinction of the contributions of goethite and nontronite was possible in Mössbauer spectra taken at 77 K. The spectral components arising from iron in the nontronite octahedral sheets could be fitted with distributions of quadrupole‐split doublets. These distributions indicate the existence of quasi‐continuous octahedral site variations rather than two discrete (cis and trans) sites, and thus favour a structural model in which all octahedral iron is in cis coordination.