Thermal Transformation of (NH4, Cs)-clinoptilolite with compositions in between the end-members

A powder containing about 90% of clinoptilolite in ammonium form, previously employed in a study concerning the thermal transformations of NH4-clinoptilolite, was used to prepare three samples with the following Cs+/NH4+ ratios: 0.51; 1.16; 5.00. These samples were subjected to thermal treatments of 2 hours up to 1200 °C. Further treatments were executed at 1100 °C for 4, 8, 16 and 32 h. (NH4, Cs)-clinoptilolite underwent dehydration, de-ammoniation, dehydroxylation followed by amorphization and crystallization of new phases. Amorphization temperature increased, from 800 to 1000 °C, with the increase of cesium content. The composition of extraframework cations played the main role in determining the type and quantity of phases in the final products, as well as the kinetics of the crystallization. Basically, the thermal transformation of (NH4, Cs)-clinoptilolite can be described by two reactions producing: i) CsAlSi5O12 (CAS) + pollucite + glass, occurred in all samples; ii) mullite + cristobalite/tridymite + glass, occurred when Cs+/NH4+ ≤ 1.16. All samples showed the lowest residual glass content after 32 h at 1100 °C. The treatment of (NH4,Cs)-clinoptilolite with Cs+/NH4+ ≥ 1.16 allowed to confine all the cesium in crystalline phases, and the residual glass was below 15%. Conversely, when Cs+/NH4+ = 0.51 most of the cesium was in the prevalent (≈ 60%) amorphous phase.