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Resilication of bauxite produced kaolin at and beneath an old erosion surface on bauxite at the Alabama Street Mine of ALCOA in Saline County, Arkansas. The transitional alteration can be traced in morphology by scanning electron microscopy, (SEM) and in Al:Si ratio by energy dispersive analysis. In one illustrated example, the sequence of resilication took place within 1 mm thickness; in another, across 80 mm. The first morphologic alteration of gibbsite (bauxite) appears to be to allophane that occurs in micrometer-size plates which show elongate cracking and/or straight to highly curved elongate edges. The next phase is kaolinite, first in micrometer-size flakes followed by coarser flakes that grade into a zone of typical stacked kaolinite, likewise identified by X-ray powder diffraction. Notably large stacks and small flakes of kaolinite are intimately mixed in the SEMs, thus suggesting that unequal sizes of kaolinite crystals can grow during one episode of an in-situ genesis. La resilification de bauxite a produit du kaolin a et sous une ancienne surface d’erosion a Alabama Street Mine d’ALCOA a Saline County, Arkansas. L’alteration de transition peut etre tracee selon la morphologie par la microscopie electronique (SEM), et selon la proportion Al:Si par analyze de dispersion d’energie. Dans un exemple illustre, la sequence de resilification s’est passee en 1 mm d’epaisseur, dans un autre, en 80 mm. La premiere alteration morphologique de la gibbsite (bauxite) semble etre en allophane qui se trouve en plaques de taille micrometrique montrant des craquelures allongees et/ou des bords allonges droits a forts courbes. La prochaine phase est la kaolinite, d’abord en paillettes de taille micrometrique, suivie de paillettes plus epaisses qui s’etagent en une zone de kaolinite empilee typique, egalement identifiee par la diffraction des rayons-X. De remarquablement larges empilements et de petites paillettes de kaolinite sont intimement melanges dans les SEM, suggerant donc que des tailles inegales de cristaux de kaolinite peuvent pousser pendant un episode de genese in-situ. [D.J.] |
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