A new deposit of Na-bentonite from the Upper Cretaceous Anacleto Formation (Neuquén Basin, Argentina): Characterization and properties

Naturally sodium bentonites are comparatively rare and the exploration of new geological sites is always of interest considering the high demand of this product. In Argentina, the predominant industrial Na-bentonites deposits are found in Neuquén Basin. Recently, in this sedimentary basin, a new Na-bentonite deposit (named Justina) has been found in the Anacleto Formation of Upper Cretaceous age and mostly deposited in fluvial-alluvial environment. The Justina deposit bentonites are constituted by around a 90% of aluminum dioctahedral smectite, identified as Na-montmorillonite. Main differences between these bentonites and the standards (Wy and CATAE) lie in a higher content of octahedral Mg, a higher BET-N2 specific surface (65–71 m2/g) and a more compacted microfabric with evidence of recrystallization and/or cementation processes affecting the smectite. Mineralogy and chemical composition so as high CEC values (111–125 cmol/kg) indicates that this benonite is of high grade. Grain size distribution, swell index, consistency limits and rheological properties of bentonites are slightly lower than expected for their composition, probably related to the occurrence of traces of soluble salts. The methylene blue adsorption (65–69 mL/0.5 g) and green compressive strength (13.9–17.4 N/m2) are high and foundry properties of the bentonites meet the requirements to be used as binder for molding sands.