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

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