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The structural chemistry of nano-FeS and ferrihydrite

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Nano-crystalline iron sulfides and oxyhyroxides are implicated in a variety of surficial terrestrial, marine, and planetary processes, including acid-mine drainage, martian diagenetic proceses, pre-biotic catalysis mimicking hydrogenase and nitrogenase, and the global iron and sulfur cycles. The composition, single-phase nature and atomic arrangements in nano-crystalline “FeS” and ferrihydrite have been controversial for at least 50 years. Contrary to previous reports, we believe both nano “FeS” and ferrihydrite are single phase, making possible interpretation of results from sorption on these substrates, intended to mimic the process and global cycles of interest. Considerable debate still surrounds the atomic arrangement in ferrihydrite in particular. A growing body of evidence confirms the composition of ferrihydrite is close to Fe10O14(OH)2. Further, a single-phase model (SPM) suffices for explanation of the atomic arrangement in inorganically and biologically derived 2- and 6-line varieties. The 50 years of effort expended on studying this material have produced many inconsistent results, probably due to variations in preparation, confusion due to over-fitting information poor (low Q, high incoherence) diffraction and x-ray absorption data with overly complex models, etc. Careful large-batch synthesis of hydrogenous and deuterated samples, and the use of several analytical probes on these same samples, provide a consistent picture.

This talk is part of the Mineral Sciences Seminars series.

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