Abstract

Scanning electron micrographs are shown of Guyana diabase altered directly to red bauxite-laterite and of Arkansas nepheline-syenite and its high-alumina weathered products. The nepheline-syenite weathers alternatively to gibbsite and/or elongate kaolin which goes to bauxite. Transported clastic Arkansas bauxite may contain pisolites, also examined by the scanning electron microscope, which are interpreted as having originated by secondary chemical rearrangement. Museum-type, classic Arkansas bauxite contains some well-crystallized, but tiny, euhedrons of gibbsite.Although bauxite develops and remains as a large-scale deposit because of the relative immobility of alumina in solution, smaller scale occurrences within the deposits show contradictory evidence of movement of alumina and crystallization from solution. Likewise, the kaolin mineral in contact with gibbsite indicates that diverse geochemical environments, as yet incompletely explained, prevailed during the bauxitization process.

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