Abstract: Mössbauer characteristics of 57Fe nuclci in 8 samples of mineral glauconite of Cambrian age and one sample of Cretaceous age (slightly disordered) were obtained at temperatures between 50 and 295 K to evaluate the site occupation of iron in the octahedral sheet and the Fe3+/Fe2+ ratio. It was found that the resolution of Mössbauer spectra of glauconite is increased at low temperature.
The octahedral sheet in mineral glauconite is believed to contain two types of cationic environments with slight symmetry differences because of the position of oxygen and hydroxyl ions around the cation sites. One environment consists of two equivalent sites (M2), whereas the other is a single site (M1).
The Mössbauer spectra were fitted to a four-doublet model, two Fe3+ sites (M2 and M1) and two Fe2+ sites, and a three-doublet model, two Fe3+ sites (M2 and M1) and an average Fe2+ site. The former (four-doublet) model yielded lower χ2 and, also, line widths in close agreement with previous studies. However, since the Fe2+ concentration in the samples is low, the error in intensity of the Fe2+ occupation of the M1 and M2 sites leads us to conclude that Fe3+/Fe2+ ratios are more accurate for the three-doublet model.
Calculation of area ratios for the two Fe3+ doublets indicates that trivalent iron is strongly concentrated in the M2 site, which is smaller than the M1 site. Site preference of divalent iron could not be determined. The ratio of Fe3+/Fe2+ obtained from the Mössbauer spectra were usually within 8% of those obtained from chemical analyses.