The partially dehydrated structure of natural heulandite: An in situ high temperature single crystal X-ray diffraction study

In situ high-temperature crystal structure analysis of natural heulandite with the chemical composition (Na_1.14 K_0.40 Ca_3.64 Sr_0.21)(Al_9.21 Si_26.88) O_72.25.2 H₂O from Maharastra, India was conducted up to 300 °C. The room-temperature and high-temperature X-ray diffraction intensities of a single crystal were measured using an imaging plate detector coupled with a U-shaped high-temperature furnace. At room temperature, the sample was monoclinic, with C2/m symmetry [a = 17.716(4) Å, b = 17.880(3) Å, c = 7.438(2) Å, and β = 116.43(1)°]. Structural refinement of the data set at room temperature yielded a final R value of R - 0.049 and R_w = 0. 115 for 1663 independent reflections. The crystal structures of the partially dehydrated states at elevated temperatures (100 °C, 150 °C, 200 °C, 250 °C and 300 °C) were refined using C2/m space group. The loss of H₂O and the accompanying migration of cations caused a change in the cell parameters: a- and c-axes and the β angle remained invariant, but the b axis decreased, leading to a reduction of the cell volume. Channels A and B became elongated and compressed, respectively, with increasing degree of dehydration. Dehydration began with the loss of H₂O coordinated to the Ca1 site in Channel A, defined by 10-membered tetrahedral rings. At 150 °C, the water coordinated to the Ca1 site was expelled radically, and it caused the Ca1 site to migrate to the cavity wall, forming a stronger bond with a tetrahedral framework, whereas the Ca2 site-coordinated water molecules remained in the 8-membered tetrahedral rings with full occupancy. The loss of water at 250 °C from Channel B triggered a structural change: Part of the structure transformed into a heat-collapsed heulandite structure with new T-O-T connections.