Évolution minéralogique et chimique des bauxites au cours de leur genèse dans les gisements de l'Ariège (France)

Abstract

The beds of bauxite of Ariège are schematically constitued, from lying wall to hanging wall, of clays with kaolinite and hematite, red bauxites with prevailing diaspore and hematite, white bauxites with prevailing boehmite and little hematite. Few red bauxites contain poorly crystallized diaspore which deshydrates at lower temperature than normal diaspore. This diaspore was synthetized by grinding gibbsite and goethite at 150 °C. The infrared spectrum suggests a hydrogen bond relatively weak which explains the lower temperature of dehydration. The different varieties of diaspore and goethite presently known are compared. The mineralogical and chemical data suggest the transformation of clays in red bauxites by leaching of only silica and crystallization of gibbsite, then the formation of white bauxite by leaching of only iron. After hiding, at approximatly 100 °C and 800 bars, diaspore forms in red bauxites and boehmite in white bauxites. In bauxites of Ariège, the iron concentration is one determinating factor in the formation of either monohydrate.