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Abstract

The Grahamstown kaolin deposits in the Republic of South Africa occur associated with two distinct geomorphological features, the Grahamstown peneplane (at about 650 m elevation and of Miocene age) and the Coastal plain (at about 520 m elevation and of some later Tertiary age). The parent rocks consist of three lithologically different shales and a diamictite of Devonian to Permian age. The clay occurs in irregularly shaped deposits having no relationship to the structure and folding or to the strati-graphic position within the sedimentary assemblages. The clay deposits are covered by thick silcrete horizons, suggesting that the deposits formed under semi-arid conditions in very flat areas having a fluctuating water table. Pyrophyllite formed prior to kaolinite in those clay deposits that developed from sediments containing both 2M1 and 1M micas as the only silicate minerals. At the beginning of weathering the less stable 1M mica appears to have favored the formation of pyrophyllite, and only at a later stage of weathering did kaolinite become more stable than pyrophyllite. The recent discovery of alunite, natroalunite, and wardite veins indicates that, beside supergene weathering, some low- to moderate-temperature hydrothermal activity may have contributed to the genesis of the kaolinitic clay deposits.

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