Dehydration and rehydration processes in gmelinite: An in situ X-ray single-crystal study

Abstract

The dehydration-rehydration process in gmelinite-Na |Na7.27K0.28Ca0.15(H2O)21.85|[Al7.71Si17.24O48]- GME, a natural zeolite that can be described as a parallel stacking of double six rings of tetrahedra in the AABB sequence, was studied by X-ray diffraction data. Its space group is P63/mmc with a = 13.764(1) and c = 10.078(1) Å cell parameters. Single-crystal data collections were performed at room conditions and at increasing temperatures, in a hot nitrogen stream, up to the fragmentation of the crystals, which occurs at a temperature as low as 100 °C, and afterward the crystal was cooled down to room conditions. X-ray powder diffraction data showed that gmelinite-Na transforms into a new structure with an AFI-type topology at about 300 °C. At room conditions, extraframework cations are located in two symmetrically independent positions, both of which are coordinated to either framework O atoms or water molecules. When the mineral is heated to 90 °C, about 40% of H2O is lost, and one cation site splits over two positions, which are three-coordinated to the framework O atoms. The dehydration process is completely reversible over a period of hours. X-ray single-crystal data has highlighted that gmelinite-Na when quenched at 100 K displays remarkable modifications in its extraframework content, resulting in a strong disorder in its extraframework ions. As in the case of heating, the mineral restores its structural features when brought back to room temperature.