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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 42 (1958)

Issue: 2. (February)

First Page: 233

Last Page: 245

Title: Argillation and Direct Bauxitization in Terms of Concentrations of Hydrogen and Metal Cations at Surface of Hydrolyzing Aluminum Silicates

Author(s): W. D. Keller (2)


Some observations have shown that certain aluminum silicate rocks weather directly to bauxite, but other observations indicate that the same rock types weather elsewhere to kaolinite which may then undergo weathering to bauxite (bauxite refers herein to a rock composed dominantly of hydrated aluminum oxide). These contrasting differences in initial products of weathering are explained by relating their separate origins to differences in pH at the surfaces of the parent minerals while undergoing hydrolysis. The pH at a mineral surface during hydrolysis is taken from the abrasion pH of the mineral.

From data of Correns, both Al2O3 and SiO2 are reported to be relatively soluble at pH above 10; at pH 7 to 9.5, Al2O3 is very sparingly soluble but silica is moderately soluble, thereby conducing toward direct bauxitization of an aluminum silicate; in the range of pH from about 7 to 4.5 the solubilities of both Al2O3 and SiO2 are low, giving rise probably to clay minerals as the products of hydrolysis of a parent aluminum silicate. Profuse rainfall conduces toward complete solution of silica and monovalent and divalent metal cations, and therefore leads toward bauxitization of minerals having a high abrasion pH (alkaline), whereas periodic and scanty rainfall, and also minerals having a relatively ow abrasion pH, tend toward the production of clay minerals during weathering. The divergence between direct bauxitization and intermediary argillation is thereby reconciled.

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