First Occurrence of Dickite in “Varicolored Clays” in the Northern Apennines (Oltrepo Pavese), Italy
F. Cellé, P. Granata, M. Setti, F. Veniale, 1993. "First Occurrence of Dickite in “Varicolored Clays” in the Northern Apennines (Oltrepo Pavese), Italy", Kaolin Genesis and Utilization, Haydn H. Murray, Wayne M. Bundy, Colin C. Harvey
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Dickite has previously been reported in “varicolored clays” of different ages in the Central and Southern Italian Apennines and in Sicily. Recently, dickite has been observed to occur randomly in Cretaceous “varicolored clays” belonging to the Scabiazza sandstone formation in the Oltrepo Pavese area of the province of Pavia (Northern Apennines).
Field observations, crystal-chemical characteristics (XRD and EDS analysis), morphological and fabric features (SEM observations) of the dickite platelets support an authigenic origin by precipitation from pore solutions. Controversial interpretations concerning the nature of such interstitial solutions, and the origin of dickite include:
diagenetic squeezing in an almost closed chemical system during tectonic events;
supergene origin in an open system, thermodynamically dependent on the physico-chemical characteristics of the solutions, which may be mixed with meteoric waters.
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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.