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Characterization of Mn oxides in cemented streambed crusts from Pinal Creek, Arizona, U.S.A., and in hot-spring deposits from Yuno-Taki Falls, Hokkaido, Japan

We report studies on cemented crusts from Pinal Creek (Arizona, USA) and on manganese hydroxide deposits found in Yuno-Taki Falls (Hokkaido, Japan), from different depositional environments. In addition to chemical analysis, contraction and expansion tests, XRD, EM, termogravimetry (TG, DTG, DSC),IR and Mössbauer spectroscopy methods were used. The most useful methods for characterizing 10 A manganese minerals were XRD, contraction test and DSC. Mössbauer spectroscopy was a very useful complementary method to XRD for the study of poorly crystalline iron compounds. We conclude that the 10 A manganates formed in both locations are similar to buserite in terms of electron microscopy characteristic and X-ray powder patterns. In resistance to dehydration, expansion and also according to an initial endotherm shown in DSC analysis, Pinal Creek crusts resemble todorokite. Using Mössbauer spectroscopy, in addition to hematite, magnetite and ilmenite, the presence of maghemite, chlorite and braunite minerals was confirmed. Based on X-ray, electron microscopy and because of the presence of birnessite (which is dehydrated buserite in air), we suggest that Pinal Creek crusts may have pre-transformation (buserite) mineralogy. From studied Yuno-Taki samples, one of them which presents relatively fresh deposit, was found to be nearly ideal buserite like our synthetic buserites. Older deposit which was exposed to water is non-ideal buserite. It can be dehydrated but not expanded. Dry deposit is buserite transformation product (birnessite or takanelite/rancieite). It contains also amorphous iron hydroxide. Autoclaved synthetic Mg-buserite transformed into a manganate with an X-ray diffraction pattern and an IR spectrum of todorokite. We suggest that this synthetic product is used as a model for high temperature hydrothermal 10 A manganate of stable todorokite structure

manganese deposits; buserite; todorokite; Pinal Creek; Yuno-Taki Falls

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