Structural Studies of Nontronites with Different Iron Contents by 57 Fe Mössbauer Spectroscopy

The 57 Fe Mössbauer spectra of a series of untreated and Ca-saturated nontronites showed a predominant Fe 3+ resonance which was computer-fitted with two Fe 3+ doublets defining iron in non-equivalent cis-FeO 4 (OH) 2 octahedral sites. In most spectra a doublet indicating tetrahedral Fe 3+ was fitted and in one untreated sample a doublet indicating interlayer Fe 3+ was identified. In a further untreated sample the interlayer iron was present as Fe 2+ . Upon Ca-saturation the interlayer iron was displaced. It also appears that the interlayer iron was present in at least two different interlayer sites. From the computer-fitted data it was clear that the interlayer cations have a significant effect on the Mössbauer resonances of iron in the two non-equivalent cis-octahedral and the tetrahedral sites of nontronite.