Iron Substitution in Montmorillonite, Illite, and Glauconite by ⁵⁷Fe Mössbauer Spectroscopy

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 Fe^2-, 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.