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The response of the laumontite crystal structure to hydration/dehydration was evaluated using Rietveld refinements with XRD data collected under controlled P H₂O  conditions at ~28.5 ℃. Refined water contents per unit cell (unit-cell formula: Ca 4 Al 8 Si 16 O 48 ·nH 2 O) ranged between 12.5 H 2 O at 0.11 mbar P H₂O  and 17.3 H 2 O at 37.6 mbar. The occupancy of the two awater sites hosting hydrogenbonded water molecules, W5 and W1, ranged from 13% to 100% and from 2% to 86%, respectively. During hydration of W5, between 0.11 and 5 mbar, the unit cell expanded continuously and reversibly from 1327 to 1348 Å 3 . The unit-cell volume remained nearly constant between 5 and 28 mbar. The hydration/dehydration of W1 exhibited hysteresis; hydration occurred at ~29 mbar and dehydration at ~24 mbar. During hydration of W1 at ~29 mbar the unit cell expanded from 1351 to 1384 Å 3 . Further hydration of W1 above 29 mbar resulted in gradual and reversible unit-cell expansion to 1386 Å 3  at 37.6 mbar. Hydration/dehydration of W5 is a continuous reaction typical for zeolites. In contrast, the hydration/dehydration of W1 at room temperature is discontinuous, as manifested by the presence of two laumontite phases during hydration and dehydration. Unit-cell parameters of the two coexisting laumontite phases observed under these conditions are consistent with a vacant W1 site and ~80%-occupied W1 site, respectively. Gradual unit-cell expansion above 29 mbar due to increased P H₂O  and increased occupancy of W1 indicate that hydration of the remaining 20% of the W1 site proceeds continuously.

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Hydrogen-bonded water in laumontite I: X-ray powder diffraction study of water site occupancy and structural changes in laumontite during room-temperature isothermal hydration/dehydration