High temperature transformation of Fe-bearing minerals in bentonite – Mössbauer spectroscopy studies

Bentonite is widely used in various branches of industry due to its special properties like low hydraulic conductivity, high sorption capacity, low permeability. High temperature stability of bentonite determines its usefulness for industrial applications. The thermal stability of the bentonite from Lower Silesia region (Wilkw, Pogrze Kaczawskie, SW Poland) has been tested at various temperatures between 100 o C and 800 o C. Raw bentonite sample contained montmorillonite (47%), magnesioferrite (30%), orthoclaste (11%) and goethite (8%). Decomposition of bentonite at high temperatures was followed by Mssbauer spectroscopy investigations. The changes in the Mssbauer spectra well reflected various phases of decomposition and transformation, because Fe-bearing minerals make up the majority (89%) in all present phases of the investigated bentonite. At a temperature of 400 o C the process of dehydroxylation of montmorillonite started, and continued up to 700 o C. The recrystallization of montmorillonite to illite was observed in a temperature above 700 o C. In a temperature range from 400 o C to 500 o C conversion of goethite to hematite took place. Mssbauer spectrum obtained after heating at 800 o C showed hematite as the main iron oxide phase with approximately the same relative ratio of iron oxides and hydroxides as in the starting material.