Birnessite-like phases occur in a wide variety of soils, and because of their submicron particle sizes, high redox activities, and layered crystal structures, they play an important role in metal cycling between soil and groundwater. The birnessite structure consists of negatively charged sheets of MnO6 octahedra that are electrostatically balanced by hydrated interlayer cations. Recent high-resolution powder diffraction analyses have demonstrated two symmetrically distinct varieties for birnessite. In the triclinic variety, as exemplified by Na-birnessite [Na0.58 (Mn4+1.42 Mn3+0.58) O4 . 1.5 H2O], the octahedral sites are fully occupied by Mn3+,4+ cations, and no Mn resides in the interlayer . In contrast, hexagonal birnessite exhibits vacancies in the octahedral sheet in lieu of Mn3+ cations, with interlayer metal cations in tridentate coordination with the basal oxygen atoms that coordinate the vacancies. The interconversion of triclinic and hexagonal birnessite has implications for processes ranging from the oxidation of Earth’s atmosphere, to the development of artificial photosynthetic materials, to the remediation of contaminated groundwater. Geochemists have proposed multiple hypotheses for the reaction pathway, but the roles of Mn oxidation state, vacancy concentration, and solution pH in promoting the transition have been poorly constrained. Using a variety of diffraction and spectroscopy techniques, we have measured concentrations of triclinic and hexagonal birnessite in suites of natural and synthetic samples, and we have explored the transition from triclinic to hexagonal birnessite as a function of pH and redox-active transition metal cations (Cr3+, Mn2+). Our results support the model that Jahn-Teller distortions associated with octahedral Mn3+ control the transition, such that hexagonal birnessite is destabilized when Mn3+ concentrations exceed ~25 mol% of total octahedral Mn. Mn3+ concentrations in turn are limited by the destabilizing effects of various interlayer cations (H+, Ca2+) on Mn3+-O octahedral bonds. The capacity of birnessite-like phases to transform reversibly between an oxidized Mn4+-rich hexagonal structure and a reduced Mn3+-rich triclinic structure raises the possibility that birnessite can function as an unconsumed intermediary in electron transport processes within soils.

Phase transitions in layered Mn oxides: Implications for electron storage and transport in soils

253rd National Meeting (2017)

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