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.