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

Two Upper Riphean glauconite samples of the Southern Urals with unusually low Fe (super 3+) /Fe (super 2+) ratios were studied by (super 57) Fe 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, Delta (sub i) (super pred) and Delta (sub i) (super tent) , for Fe (super 3+) and Fe (super 2+) , respectively, in different local cationic arrangements (LCAs) with their occurrence probabilities, w (sub i) , provided by two-dimensional simulation of the cation distribution (CD). Using the parameters of the crystal-chemical model, QSD (super sim) profiles were simulated and compared with the experimental QSDs. To obtain agreement between QSD (super exp) and QSD (super sim) profile, w (sub i) and Delta (sub i) values were adjusted using the method of successive approximations. As a result, the tentative Delta (sub i) (super tent) values for Fe (super 2+) contributions to Mossbauer spectra were refined. The sequence of refined Delta (sub i) (super ref) values corresponding to specific LCA types remained the same as that for the Delta (sub i) (super tent) values. For the studied glauconites, the CD reconstructions satisfying the experimental QSDs are characterized by similar local distribution of cations around Fe (super 3+) and by different local cation distribution around Fe (super 2+) .