White bentonite from Patagonia, Rio Negro Province, Argentina

摘要

The exploitation of white and non-swelling bentonite whose geological and mineralogical features were not widely known, has recently begun in Rio Negro Province, in South of Argentine. The deposits are alteration products of pyroclastic materials (volcanic ash) that originated in late Cretaceous volcanic events. These deposits are composed mainly of montmorillonite and disordered cristobalite-tridymite (opal-CT). Other impurities and accessory minerals are glass shards, quartz, albite, calcite, K-feldspars, biotite, pyrolusite, pyroclastic rock fragments and diatom shells. The whiteness index reaches 78%. Their CEC varies between 83 and 98 meq/100g. Chemical composition shows a low Fe_2O_3 content that varies between 0.80 and 0.85% and a moderately high magnesium content: 3.5 and 3.9%. The main octahedral cation is aluminum. Calcium plus magnesium predominate over sodium as natural interlayer cations. The replacement of Si by Al is very low and the main part of the layer charge comes from substitutions in the octahedral sheet. White bentonites like these, are relatively rare in occurrence, compared to other bentonites, particularly in Argentina. Technologically, this bentonite has rheological properties that do not favor its use as a component in drilling muds and gels. Nevertheless, it shows interesting properties for it use as plasticisers in ceramic and refractories, as a filler and strengthener in plastics, as a filler in rubber, as a decolorizing (bleaching) and refining agent for oils, and many other applications.