Five different samples of (Ba, Sr)-exchanged zeolite A, with a residual Na content ranging between 0.14 and 0.23 meq g_1, were prepared by cation exchange so as to cover the whole composition range between the pure Ba- and Sr-end terms. In order to investigate the thermal conversion of these samples into monoclinic celsian solid solutions, thermal treatments in the temperature range 200–1200 °C for times up to 5 h were performed and the obtained products characterized by room temperature XRD. The results of these investigations were discussed in the light of previous studies concerning the thermal transformations of Ba-exchanged and Sr-exchanged zeolite A. Interesting results were obtained. In particular the total conversion of a sample of (Ba, Sr)-exchanged zeolite A, with a Sr2+ equivalent fraction of 0.14 and a Na+ residual content of 0.18 meq g_1, in the monoclinic polymorph of (Ba, Sr)Al2Si2O8 solid solution was attained by a 5 h thermal treatment at 1100 °C.
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