Sem Investigation of a Lateritic Weathering Profile, Saline County, Arkansas
Timothy L. Salter, Haydn H. Murray, 1993. "Sem Investigation of a Lateritic Weathering Profile, Saline County, Arkansas", Kaolin Genesis and Utilization, Haydn H. Murray, Wayne M. Bundy, Colin C. Harvey
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The morphologies, textures, and compositions of minerals and amorphous materials from a saprolite located in Saline County, Arkansas were examined by scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS) supplemented by X-ray powder diffraction (XRD). As shown by Walter Keller in a landmark series of papers, SEM/EDS analysis is necessary (in conjunction with XRD) to reveal complex textural sequences and paragenetic relationships that are crucial to understanding saprolite genesis. Of particular interest are three amorphous materials identified by their distinctive textures and compositions. The three abundant amorphous textural types are likely composed of mixtures of allophane and other amorphous materials. These amorphous weathering products were precursors to halloysite, goethite, and other minerals of the saprolite. The saprolite also contains an illite-like (10 Å) mineral. The gradational boundary between the saprolite and the overlying kaolinite-rich clay is marked by a switch from a system that produced principally kaolinite to one that formed halloysite.
A model of saprolite genesis is developed based on previous experimental and theoretical investigations. The observed sequence of amorphous materials and secondary minerals can be related to the coordination of aqueous aluminum and to solution saturation, both of which are pH dependent. The role of A1 coordination in the crystallization of kaolinite and halloysite is not clearly defined, but experimental evidence suggests that halloysite may form at higher aqueous A1IV:A1VI ratios than kaolinite. Illite only occurs in saprolite beneath a continuous layer of iron oxyhydroxide suggesting that restricted ground water flow allowed illite to precipitate.
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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.