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

The dehydration-rehydration process in gmelinite-Na |Na(7.27)K(0.28)Ca(0.15)(H(2)O)](21.85)|[Al(7.71)Si(17.24)O(48)]-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 P6(3)/mmc with a = 13.764(1) and c = 10.078(1) angstrom 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 degrees 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 degrees 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 degrees C, about 40% of H(2)O 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.