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Three gray kaolin sites in Georgia were cored to investigate by X-ray diffraction, radiography, fluorescence and scanning electron microscopy the processes involved in kaolin deposit formation. Kaolinite crystallinity increased and mica content decreased in those regions of the cores containing abundant kaolinite vermiforms. Excess total Fe above that allocated to sulfides correlated well with K content in all reduced kaolins sampled. Allocation of the Fe in the deposit based on the K-Fe correlation suggests that much of the Fe in gray kaolins is present in an Fe-bearing mica impurity. Calculation of the Fe content of the mica using the K as a measure of mica content yields an Fe concentration of 2.9% as Fe2O3 for the mica in samples from both the Cretaceous and Tertiary kaolins. This concentration corresponds to 1 octahedral site in 14 filled by Fe in muscovite. If the remaining Fe is allocated to kaolinite, it would contain 0.15% Fe2O3 (i.e. 1 of every 400 octahedral sites contains Fe). The results of this investigation support a hypothesis that during the development of kaolin deposits Fe is released by the dissolution of mica and is leached or precipitated as sulfides while some of the kaolinite is altered to vermiforms of higher crystallinity.

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