State analysis of iron in allophanic clays. I. Moessbauer effect analysis of iron in allophanic clays.

概要

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In order to investigate the state of iron in naturally occurring allophanic clays, Mossbauer resonance absorption spectra were measured for allophanic clays and allophanes together with reference samples of imogolite and hisingerite (amorphous iron silicate). The Mossbauer spectra of allophanic clays gave a sharp ferric quadrupole doublet, which strongly suggests iron in a relatively uniform octahedral coordination. The Mossbauer parameters (quadrupole splitting and isomer shift) of Fe3+ in allophanic clays and allophanes were significantly different from those of hisingerite which had been reported by Kohyama and Sudo (1975) as a very poorly crystalline form of nontronite. The appearance of poor but apparently superimposed spectra consisting of ferric and ferrous quadrupole doublets in allophane strongly suggested that iron still remained in part in the above-mentioned octahedral coordination, although dithionite-citrate treatment caused the change in the coordinated state of iron involved. However, iron in a uniform coordination could not be detected for imogolite. Some discussion on the form of iron (Fe3+) in allophane and allophanic clays was made by comparing Mossbauer parameters of iron in these clays with those of iron in some other naturally occurring iron-containing minerals.
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