Solid-state transformation of (NH4, Ba)-clinoptilolite to monocelsian, mullite, and cristobalite

Three clinoptilolite-based materials, with Ba2+/NH4+ ratios of 2.27, 0.60 and 0.11, were prepared starting from a powder containing about 90% of NH4-clinoptilolite. The samples were submitted to thermal treatments of 2 h up to 1200 °C. Further treatments were carried out at 1100 and 1200 °C for 4, 8, 16 and 32 h. The sequence of reactions observed in heating (NH4, Ba)-clinoptilolite is: dehydration; de-ammoniation (along with residual dehydration); dehydroxylation; amorphization; crystallization of new phases. Amorphization temperature varied from 700 to 900 °C, and increased with barium content. Monocelsian, mullite and cristobalite/tridymite were obtained from all samples heated 32 h at 1100 °C, along with residual glass that never exceeded 9%. The Ba2+/NH4+ ratio resulted the main factor in determining type and quantity of the phases formed in the fired products, as well as their order of crystallization, but also temperature and duration of treatment affected the result. At 1100 °C mullite crystallized as last phase in the Ba-rich sample, and it did not form at 1200 °C. Similarly, celsian was never obtained at 1200 °C from the ammonium-rich material, and was the last phase to crystallize at 1100 °C. Transformations were faster at 1200 °C, but the amounts of residual glass were higher. Hexacelsian, the metastable polymorph of Ba-feldspar which is always formed during the heating of Ba-exchanged zeolites A and X, was never recorded among crystallization products obtained from (NH4, Ba)-clinoptilolite.