The research of W. D. Keller was influential in resolving questions concerning the origin of the Georgia-South Carolina kaolin deposits. Keller's research on the factors controlling the formation of kaolinite is the cornerstone for our understanding of the origin of these unique kaolin deposits. The development of the ideas central to our understanding of these deposits are outlined in this paper.
The present view of the origin of the soft kaolin deposits is summarized as follows. Kaolinite was formed by the alteration of feldspar and mica in coarse grained metamorphic and igneous rocks of the Piedmont Plateau and associated arkosic sands. Kaolinitic sediment from the crystalline rocks was carried southeast toward the Cretaceous shoreline. Subsequently, rapid erosion of non-altered crystalline rocks led to the deposition of arkosic sediments with partially altered feldspars. Repeated exposure to the harsh weathering conditions resulted in the alteration of most of the feldspar to kaolinite. The transportation process led to the natural separation of significant amounts of quartz, feldspar, and mica from kaolinite. The kaolin rich sediments were deposited in fresh to slightly brackish quiet water environments associated with fluvial-deltaic settings at the Cretaceous shore. Deposition of kaolinite in fresh water induced face-to-edge flocculation and an open pore structure. Continued rapid addition of fluvial sediment quickly covered the fine material in the sedimentary sequence. Subsequently, groundwater altered the remaining feldspar to kaolinite and this facilitated the formation of large vermicular crystals.
The hard kaolin deposits formed in a different manner. The sea level changes during the late Cretaceous to early Tertiary eventually exposed fine grained metavolcanic rocks of east Georgia and South Carolina to intense weathering conditions. Under these conditions kaolinite formed from fine grained mica in phyllite and fine grained schist. The kaolin rich sediments were transported to the Eocene shoreline and were deposited in brackish to marine environments. Deposition in saline water caused face-to-face flocculation and resulted in a tightly packed sediment that was not easily altered by groundwater.
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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.