Abstract

Two Upper Riphean glauconite samples of the Southern Urals with unusually low Fe3+/Fe2+ ratios were studied by 57Fe Mössbauer and IR spectroscopy. Room and liquid-nitrogen temperature Mössbauer spectra were analyzed with quasi-continuous model-independent quadrupole splitting distributions (QSDs). The obtained QSDs were interpreted using a crystal-chemical model taking account of the effects of the local structural and chemical heterogeneity of layer silicates. The main parameters of the crystal-chemical model are individual quadrupole splittings, Δipred and Δitent, for Fe3+ and Fe2+, respectively, in different local cationic arrangements (LCAs) with their occurrence probabilities, wi, provided by two-dimensional simulation of the cation distribution (CD). Using the parameters of the crystal-chemical model, QSDsim profiles were simulated and compared with the experimental QSDs. To obtain agreement between QSDexp and QSDsim profile, wi and Δi values were adjusted using the method of successive approximations. As a result, the tentative Δitent values for Fe2+ contributions to Mössbauer spectra were refined. The sequence of refined Δiref values corresponding to specific LCA types remained the same as that for the Δitent values. For the studied glauconites, the CD reconstructions satisfying the experimental QSDs are characterized by similar local distribution of cations around Fe3+ and by different local cation distribution around Fe2+.

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