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Abstract

The mineralogy and geochemistry of limonite from the Middle Eocene Lone Star iron ores of east Texas were investigated. X-ray powder diffraction analysis indicates that the major minerals are goethite, kaolinite, and quartz, with minor amounts of lepidocrocite. Electron microprobe data show an excellent correlation (r = .98) between A12O3 and SiO2, with both oxides inversely proportional to Fe2O3 content. These relationships indicate a mixture of goethite and kaolinite. All the Al in the limonite ore can be accounted for by the kaolinite, suggesting that goethite does not contain appreciable amounts of Al. The degree of Al substitution in goethite as determined by the position of the 111 goethite reflections indicates that Al substitution is < 5%, in agreement with the conclusion based on the microprobe data. The meager Al substitution in goethite and the presence of lepidocrocite indicates that the Lone Star limonite formed in a hydromorphic environment. Two processes are responsible for the precipitation of goethite; in situ oxidation of primary iron-rich minerals (berthierine and siderite) and oxidation of Fe+2 in solution.

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