Comparison between the quasi-continuous quadrupole splitting distributions (QSD) for Mossbauer spectra of glauconites and the QSD-profiles simulated on the basis of crystal-chemical model

摘要

Structural peculiarities of Fe~(3+)-rich dioctahedral mica, glauconite, such as a wide variety of isomorphous octahedral cation substitutions (Al, Fe~(3+), Fe , Mg) and the occurrence of these cations in cis-sites only, make it possible to consider glauconites as model locally inhomogeneous objects. To fit room and liquid-nitrogen Mossbauer spectra of two glauconite samples of close compositions with unusually low Fe~(3+)/Fe~(2+) ratios, reconstruction of quadrupole splitting distributions (QSD) in terms of quasi-continuous model-independent approach was applied. Interpretation of the obtained QSD~(exp)s became possible owing to application of a crystal-chemical model (CCM) taking account of the effects of the local structural and chemical heterogeneity of layer silicates. The main parameters of the CCM are individual quadrupole splittings, ⊿_i, for Fe~(3+) and Fe~(2+) in different local cationic arrangements (LCAs) having the occurrence probabilities, w_i provided by two-dimensional simulation of the cation distribution (CD). The agreement between CD reconstructions, the QSD~(slm) profiles simulated on the basis of the CCM parameters and the QSD~(exp)s provided, first, the corroboration of the ⊿_i~(pred) values for Fe~(3+); second, the refinement of the ⊿_i~(ref) values for Fe~(2+); and third, revealing the difference in the octahedral cation distributions for the studied glauconites.