The Geology, Mineralogy and Exploitation of Halloysite Clays of Northland, New Zealand
Colin C. Harvey, Haydn H. Murray, 1993. "The Geology, Mineralogy and Exploitation of Halloysite Clays of Northland, New Zealand", Kaolin Genesis and Utilization, Haydn H. Murray, Wayne M. Bundy, Colin C. Harvey
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In Northland, New Zealand, several large deposits of hydrated halloysite clay have been formed by the in situ alteration of rhyolite volcanics of Pliocene or Pleistocene age. The rhyolites were erupted through pre-Pliocene sedimentary strata and locally through basalt. They are onlapped in places by more recent basalt flows. The halloysite clays have been formed from both hydrothermal alteration and sub-tropical weathering.
The clays are mined by open-pit methods and processed by crushing and fractionation to a product of 98% finer than 2 μm particle size, which is of exceptional whiteness and brightness. The unique properties of these halloysites has led to their world wide utilisation in specialised ceramic applications.
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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.