A Mössbauer Investigation of Glauconite and its Geological Significance

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 Fe 3+ and Fe 2+ ions at room temperature. The low temperature spectra indicate that all Fe in the glauconites is in octahedral coordination. Fe 3+ and Fe 2+ ions occur in both eis and trans sites; Fe 3+ shows a strong preference for eis sites whereas Fe 2+ 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 Fe 2+ for other cations which temporarily stabilize the progenitor, followed by Fe 2+ -Fe 3+ 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 Fe 3+ and its Fe 2+ content when placed in a solution containing Fe 2+ as the only Fe ion present.