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

J. H. Johnston and C. M. Cardile1
Chemistry Department, Victoria University of Wellington Private Bag, Wellington, New Zealand
1 Present address: Chemistry Division, Department of Scientific and Industrial Research, Private Bag, Petone, Wellington, New Zealand.

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.

Key Words: Glauconite • Illite • Iron • Montmorillonite • Mössbauer spectra

Clays and Clay Minerals; June 1987 v. 35; no. 3; p. 170-176; DOI: 10.1346/CCMN.1987.0350302
© 1987, The Clay Minerals Society
Clay Minerals Society (www.clays.org)