Abstract
Two occurrences of bentonite beds have been studied in Aceguá (RS, Brazil) and Melo (Uruguay). Despite their different thicknesses, the two profiles seem to belong to the same bentonite bed because of their similar petrographical and mineralogical properties (color, density, composition). Both are composed of a nearly pure smectite mineral that is dominantly a montmorillonite. High amounts of Mg ions in the octahedral sheet elevate the layer charge approximately 0.5 per Si 4O 10. The layer charge is mostly compensated for by Ca 2+ ions in the interlayer. The rare earth element (REE) patterns are different from the post-Archean Australian average shale (PAAS) standard. In particular, when the data are PAAS normalized, the pattern obtained shows that the bentonite is LREE depleted. Thus, it cannot have originated from detrital sources. Despite diagenetic conditions that favor the crystallization of random illite/smectite mixed layers from detrital clays in the surrounding formations, the mineral reactions in the bentonite bed are limited to a few Al for Si substitutions in the tetrahedral sheets. The clays remain dominantly montmorillonite with abnormally high crystallinity.
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