A Mössbauer Investigation of Glauconite and Its Geological Significance

D. M. McConchie1, J. B. Ward2, V. H. McCann3 and D. W. Lewis4
1 Department of Geology, University of Geology, Christchurch, New Zealand. Present address: Geology Department, University of Western Australia, Nedlands, Western Australia 6009, Australia.
2 Department of Physics, University of Canterbury, Christchurch, New Zealand.
3 Department of Physics, University of Canterbury, Christchurch, New Zealand.
4 Department of Geology, University of Canterbury, Christchurch, New Zealand.

Abstract: Mössbauer spectra of 9 glauconite samples from Upper Cretaceous and Lower Tertiary strata in the South Island of New Zealand contain a broad shoulder due to low intensity absorption continuous between 1.0 and 2.5 mm/sec when the absorber is at room temperature; the shoulder is absent, and sharp peaks are apparent in spectra taken with the absorber at 80°K. The data suggest that electron transfer occurs between adjacent Fe3+ and Fe2+ ions at room temperature. The low temperature spectra indicate that all Fe in the glauconites is in octahedral coordination. Fe2+ and Fe2+ ions occur in both cis and trans sites; Fe3+ shows a strong preference for cis sites whereas Fe2+ shows an even stronger preference for trans sites.

The partially variable oxidation state of Fe in glauconite is interpreted in terms of a geochemical model for glauconitization of a degraded or incomplete progenitor phyllosilicate. The model involves exchange of Fe2+ for other cations which temporarily stabilize the progenitor, followed by Fe2+–Fe3+ charge transfer reactions. Each reaction results from the system's tendency towards equilibrium. The model is supported by the observation that artificially leached glauconite increases both its Fe3+ and its Fe2+ content when placed in a solution containing Fe2+ as the only Fe ion present.

Key Words: Genesis • Glauconite • Iron oxidation • Mössbauer spectroscopy

Clays and Clay Minerals; October 1979 v. 27; no. 5; p. 339-348; DOI: 10.1346/CCMN.1979.0270504
© 1979, The Clay Minerals Society
Clay Minerals Society (www.clays.org)