The most prevalent natural phyllomanganates, and most extensively studied synthetic layer Mn oxide phases, are those having the birnessite/buserite-like structure, and are constructed of sheets of Mn4+/Mn3+-O octahedra with ~7 Å (birnessite) or ~10 Å (buserite) basal spacing. This seemingly simple structure challenges researchers with surprising variations and complexities. The interlayer region, for example, is occupied by one or more types of hydrated cations, e.g. Na+, Ca2+, Ba 2+, K+, Mn 2+ etc., which are readily exchangeable, and the Mn octahedral sheets consist of various amounts of Mn4+, Mn3+ , and vacancies. Transformations between “hexagonal” and triclinic structures readily occur in response to changes in pH and redox conditions, or cation exchange, etc. Consequently, myriad natural or synthetic, biotic or abiotic, processes, responding to a variety of conditions, and subsequent treatments, can yield a large assortment of so-called “birnessites”. We have characterized the structures and behaviors of a wide range of natural and synthetic, and abiotic and biotic, birnessite-like phases using time-resolved synchrotron powder XRD, neutron diffraction, analytical SEM, XPS, and EXAFS/XANES, FTIR and Raman spectroscopies. The spectroscopic methods determine Mn oxidation state, i.e. Mn4+:Mn3+:Mn2+, (XANES and XPS) and provide structural insights for poorly crystalline samples. In particular, FTIR and Raman spectroscopies have proven effective at revealing information about structure symmetry, i.e. triclinic versus hexagonal. A series of experiments that systematically synthesized birnessite-like phases elucidated how particular interlayer cations affect crystallinity and symmetry. This study provides new insights into structural variations and complexities of birnessite-like phases as functions of composition and formation environment.

Structures and behaviors of birnessites

255th National Meeting (2018)

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