Iron and manganese minerals from South African ironstone deposits

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Ironstone deposits are common in the Barberton Mountain Land of South Africa. They are composed of fine-grained goethite with significant amounts of several manganese oxide phases. Fossil bacteria in these rocks are often preserved within the iron and manganese oxide phases, and bacteria may have played a significant role in the formation of the deposits. Manganese minerals, in particular lithiophorite (LiAl (Mn Mn )O (OH) ) and todorokite ((Ca, Na, K) (Mn Mn ) O •3.5H O), have been identified by x-ray diffractometry. These rocks have now been studied by x-ray absorption fine structure spectroscopy. X-ray absorption near edge structure (XANES) spectroscopy showed that the oxidation state of both iron and manganese in these rocks is high. Least squares modeling of the x-ray absorption near edge structure (XANES) spectra shows that most of the manganese is present in the oxidation states +III and +IV, with no detectable manganese as +II. Similarly for iron, only +III oxidation state could be identified in most rocks. Extended x-ray absorption fine structure (EXAFS) studies confirm that both edge-sharing (lithiophorite, todorokite) and corner-sharing octahedra (todorokite) are present and constrains the proportions of these two minerals. Bacteria stabilized the higher oxidation states of iron and manganese during precipitation, acting as an important component of the mineralization. © Physica Scripta 2005. 2 2 6 6 x 6 12 2 +IV +III +IV +III

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Physica Scripta T

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