Recrystallization of Kaolinite in Gray Kaolins
G. Norman White, J. B. Dixon, R. M. Weaver, A. C. Kunkle, 1993. "Recrystallization of Kaolinite in Gray Kaolins", Kaolin Genesis and Utilization, Haydn H. Murray, Wayne M. Bundy, Colin C. Harvey
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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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Kaolin is an important industrial mineral in several world markets including uses in paper coating and filling, ceramics, paint, plastics, rubber, ink, fiberglass, cracking catalysts and many other uses (Murray, 1991). The kaolin minerals kaolinite, halloysite, dickite, and nacrite have essentially similar chemical composition but each has important structural and stacking differences. The most common kaolin mineral and the one that is the most important industrially is kaolinite [Al2Si205(OH)4]. Kaolinite can be formed as a residual weathering product, by hydrothermal alteration, and as an authigenic sedimentary mineral. The residual and hydrothermal occurrences are classed as primary and the sedimentary occurrences as secondary. Primary kaolins are those that have formed in situ usually by the alteration of crystalline rocks such as granites and rhyolites. The alteration results from surface weathering, groundwater movement below the surface or action of hydrothermal fluids. Secondary kaolins are sedimentary which were eroded, transported and deposited as beds or lenses associated with other sedimentary rocks. Most kaolin deposits of secondary origin were formed by the deposition of kaolinite which had been formed elsewhere. Some secondary deposits were formed from arkosic sediments that were altered after deposition, primarily by groundwater. There are far more deposits of primary kaolins in the world than secondary kaolin deposits because special geologic conditions are necessary for both the deposition and preservation of secondary kaolins.