Genetic Significance of Paramagnetic Centers in Kaolinites
Paramagnetic impurities, Fe3+, Mn2+ and vanadyl (VO2+) ions, along with paramagnetic radiation-induced defect centers, trapped holes, have been investigated with electron paramagnetic resonance (EPR) in more than 350 kaolin samples from different localities. The trace elements determined are either substituted (Fe3+) or adsorbed (Mn2+ and VO2+). The thermal stability of the radiation-induced defects has been determined. The genetic significance of the EPR spectra has been considered on the basis of previous data from the literature and new informations obtained in this study. It is shown that paramagnetic centers: (1) can be used to differentiate kaolinites from the three major environments at the Earth’s surface, hydrothermal, weathering and sediments; (2) are indicators of growth conditions of kaolinite; (3) provide a basis for interpreting conditions of kaolinite formation in different environments; and (4) fingerprint accurately several stages of kaolinite growth and/or successive geochemical events.
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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.