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.