Effects of Iron Oxidation State on the Texture and Structural Order of Na-Nontronite Gels

Joseph W. Stucki1 and Daniel Tessier2
1 Department of Agronomy, University of Illinois, Urbana, Illinois, 61801
2 Station de Science du Sol, INRA, Route de Saint Cyr, 78026 Versailles, France

Abstract: Aqueous gels of unaltered (oxidized) and chemically reduced ferruginous smectite (SWa-1 from the Source Clays Repository of The Clay Minerals Society) were characterized by transmission electron microscopy, electron diffraction, and energy-dispersive X-ray fluorescence to establish details regarding their texture, inter-layer and inter-particle arrangements, and chemical composition. Micrographs revealed that the reduction of structural Fe(III) to Fe(II) caused a consolidation of smectite particles from an extensive network of small crystals (1–6 layers thick) to distinct particles of limited size in the a–b direction and about 20–40 layers thick. The interlayer distances in the reduced sample appeared to be more uniform than in the oxidized sample, but both exhibited spacings of about 12.6 Å. Chemical analysis showed no qualitative differences as a result of oxidation state. Electron diffraction patterns displayed marked differences. The pattern of the oxidized sample consisted of homogeneous rings, indicating that the stacking order in the a–b plane was turbostratic or disordered, whereas the reduced pattern exhibited much more order as evidenced by distinct spots amid low-intensity rings, suggesting that inter-layer attractive forces were stronger if Fe(II) was present in the clay crystal.

Key Words: Electron diffraction • Ferric iron • Ferrous iron • High-resolution transmission electron microscopy • Layer structure • Nontronite

Clays and Clay Minerals; April 1991 v. 39; no. 2; p. 137-143; DOI: 10.1346/CCMN.1991.0390204
© 1991, The Clay Minerals Society
Clay Minerals Society (www.clays.org)