Abstract: The 57Fe Mössbauer spectra of an iron-rich montmorillonite, an illite, and two glauconites were measured and computer-fitted with appropriate Fe3+ and Fe2+ doublet resonances. The broad experimental Fe3+ resonance of montmorillonite probably arises from Fe3+ in the octahedral sites and a trans-arrangement of OH groups; however, a large variation in the neighboring environment of these sites exists. In illite this Fe3+ resonance is similar but shows less broadening; it arises from Fe3+ located predominantly in trans-OH octahedral sites, with some Fe3+ being located in cis-OH octahedral sites. Because of the increased iron content less variation exists, compared with montmorillonite, in the neighboring octahedral sites. The Fe3+ resonance is narrower still for the glauconites and represents Fe3+ substituting primarily into cis-OH octahedral sites, similar to that previously reported for nontronite.
The tetrahedral Fe3+ content is very low for montmorillonite and increases progressively for illite and glauconite, suggesting that a higher tetrahedral Fe3+ content directs Fe3+ in the octahedral layer into cis-OH sites. In montmorillonite, the Fe2+ is located only in trans-OH sites; in illite Fe2+ is largely in trans-OH sites and only slightly in cis-OH sites; and in glauconite, Fe2+ is located largely in cis-OH sites and only slightly in trans-OH sites. These assignments suggest that for Fe2-, the doublet with the larger quadrupole interaction arises from Fe2+ in trans-OH sites and the doublet with the smaller quadrupole interaction, from Fe2+ in cis-OH sites.