An improved model for the interpretation of Mossbauer spectra of dioctahedral 2:1 trans-vacant Fe-rich micas: refinement of parameters

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Two Upper Riphean glauconite samples of the Southern Urals with unusually low Fe3+/Fe2+ ratios were studied by 57Fe Mossbauer and IR spectroscopy. Room and liquid-nitrogen temperature Mossbauer 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, Δ i pred and Δ i tent, for Fe3+ and Fe2+, respectively, in different local cationic arrangements (LCAs) with their occurrence probabilities, w i , 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, w i and Δ i values were adjusted using the method of successive approximations. As a result, the tentative Δ i tent values for Fe2+ contributions to Mossbauer spectra were refined. The sequence of refined Δ i ref values corresponding to specific LCA types remained the same as that for the Δ i tent 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+.