Thermal behavior of stilbite and stellerite revisited and dehydration of their Na-exchanged forms: Considerations on the memory effect of the STI framework type

Abstract

The thermal behavior of Na-exchanged stellerite and stilbite was investigated by in-situ single crystal X-ray diffraction. For comparison with the exchanged forms new data were collected on natural stellerite and stilbite under the same experimental conditions. With the increase of temperature, strong disorder at T and O sites of the tetrahedra of the four-membered ring developed in natural forms. Such disorder was associated with the rupture of T-O-T connections and transition from the A to the B phase. Differently from previous studies, stellerite B at T > 300 °C was found to be monoclinic (space group A2/m). In addition, at 400 °C, a new T-O-T connection occurred, analogous to that in the B phase of barrerite.Na-stellerite and Na-stilbite were at RT monoclinic, space group F2/m. Upon heating, they also displayed the same structural modifications as observed in natural barrerite and Na-barrerite and adopted space group A2/m. Compared to natural stellerite and stilbite different T-O-T connections ruptured leading to a different topology of the B phase. The total volume contraction was −16% at 350 °C compared to −8% of pristine materials. The highly-condensed D phase, which does not form in natural stellerite and stilbite, was obtained by heating a Na-stellerite crystal ex-situ at 525 °C. The structure corresponded to the D phase of natural barrerite and Na-barrerite.All investigated STI members, after being exchanged with Na, have identical symmetry and demonstrate corresponding behavior upon heating and associated dehydration. Thus, a previously assumed memory effect of the symmetry of the natural parent structure, is not confirmed.