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

Abstract

The response of the laumontite crystal structure to hydration/dehydration was evaluated using Rietveld refinements with XRD data collected under controlled PH2O conditions at ~28.5 °C. Refined water contents per unit cell (unit-cell formula: Ca4Al8Si16O48·nH2O) 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 Å3. 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 Å3. Further hydration of W1 above 29 mbar resulted in gradual and reversible unit-cell expansion to 1386 Å3 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 PH2O and increased occupancy of W1 indicate that hydration of the remaining 20% of the W1 site proceeds continuously.