A Sr-exchanged heulandite crystal of composition Sr4.35Ca0.13(Al8.96Si27.04O72)·26H2O was used for stepwise dehydration experiments. The crystal was heated for approximately 12 h from room temperature to 250 °C in steps of 50 °C using an airflow-heater device. For single-crystal X-ray data collection the crystal was quenched to −173 °C with liquid-N2 on the diffractometer. Due to pronounced Sr order deviating from the topological symmetry C2/m, the structure was refined in space group Cm for each dehydration state. The initial H2O content of 26 molecules per formula unit (pfu) at room temperature decreased to 17 molecules pfu after heating at 250 °C. Heating to 270 °C mechanically destroyed the crystal and a completely dehydrated state could not be studied. The loss of H2O and accompanying migration of Sr caused a change of cell parameters: a and c slightly decreased, b decreased, and β remained invariant, leading to a reduction of the cell volume. As Sr loses H2O upon dehydration, it moves toward the C rings and forms stronger bonds to the tetrahedral framework. With increasing dehydration the A and B ring become slightly compressed and elongated. Initially highly populated Sr sites split into less populated sites caused by the loss of surrounding H2O molecules.

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