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

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 similar to28.5 degreesC. Refined water contents per unit cell (unit-cell formula: Ca4Al8Si16O48.nH(2)O) ranged between 12.5 H2O at 0.11 mbar PH2O and 17.3 H2O at 37.6 mbar. The occupancy of the two water sites hosting hydrogen-bonded 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 Angstrom(3). The unit-cell volume remained nearly constant between 5 and 28 mbar. The hydration/dehydration of W1 exhibited hysteresis; hydration occurred at similar to29 mbar and dehydration at similar to24 mbar. During hydration of W1 at similar to29 mbar the unit cell expanded from 1351 to 1384 Angstrom(3). Further hydration of W1 above 29 mbar resulted in gradual and reversible unit-cell expansion to 1386 Angstrom(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 W I site and similar to80%-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.