Kaolins, Kaolins, and Kaolins
Kaolin is an important industrial mineral and geological indicator. It occurs in hydrothermal, residual and sedimentary deposits. The first two are classed as primary occurrences and the third as secondary. The physical and chemical conditions under which kaolins form are at relatively low temperatures and pressures. The most common parent rocks are granites and rhyolites and the most common parent minerals are feldspars and muscovite. The kaolin minerals are kaolinite, halloysite, dickite and nacrite and by far the most common is kaolinite.
The physical and chemical properties of kaolin determine its ultimate utilization. Some kaolins can be used as paper coating clays, some as filler clays in several industries, some for ceramics and refractories and some for special uses. Important kaolin deposits that are briefly described are those in Australia, Brazil, China, Czechoslovakia, England, Germany, Indonesia, Japan, Mexico, New Zealand, Spain, Ukraine, and the United States.
The uses of kaolins are governed by several factors including the geological conditions under which the deposits formed, their mineralogical composition and physical, chemical and optical properties. The variable properties and uses and the differing occurrences and mineralogy explains why the title “Kaolins, Kaolins and Kaolins” was selected.
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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.