A STRUCTURAL AND THERMAL STUDY OF HYDRATION/DEHYDRATION IN LAUMONTITE

Laumontite is a common zeolite occurring in geothermal systems between ~100 to ~200 °C. Fully hydrated laumontite contains 4.5 moles of H2O per formula unit of 12 framework O (pfu); 3 are located on W2 and W8, 1 on W1 and 0.5 on W5. Hydration/dehydration equilibria between laumontite and water vapor were investigated by TGA and XRD under controlled PH2O to characterize the properties of water on the W1 and W5 sites. Reversed isothermal TGA experiments at 24 °C showed that hydration/dehydration occurs in discrete steps; between 0 and 4 mbar PH2O, 0.4 H2O pfu are gained/lost, and between 19.5 and 24 mbar PH2O 0.8 H2O pfu are gained/lost. Hydration/dehydration is reversible at 0 to 4 mbar PH2O but exhibits hysteresis at 19.5 to 24 mbar PH2O. Rietveld crystal structure refinements using XRD patterns collected at 0.7, 16, and 40 mbar PH2O confirmed that the hydration/dehydration step between 0 and 4 mbar PH2O involves only the W5 site and that the hydration/dehydration step at 19.5 to 24 mbar PH2O involves only the W1 site. XRD experiments under variable PH2O showed that changes in the crystal structure associated with the loss/gain of water on the W5 site are continuous but those associated with the loss/gain of water on the W1 site are discontinuous. Unit-cell refinement using XRD patterns collected during the hydration of W1 confirms the presence of two laumontite phases in the sample, one with unit-cell consistent with an empty W1 site, and the other one consistent with a nearly fully occupied W1 site. Water adsorption isotherms between 0 and 30 mbar PH2O were collected at 41.1, 60.8 and 80.5 °C. These isotherms exhibited “reversed” hysteresis. Hydration/dehydration under these conditions involved water on W5 only. These isotherms were fitted with the Langmuir equation and a single parameter mixing model to account for excess energy of mixing. The results constrain the thermodynamic properties of the water adsorption reaction on W5 allowing evaluation of the thermodynamic properties of zeolitic water on that site. The enthalpy associated with hydration of the W5 site is more negative than that of the W1 site by ~ 7 KJ/mole of H2O, and less negative than that for the W2, W5, and W8 by ~26 KJ/mole of H2O (c.f. Kiseleva et al., 1996). Our results predict that the W5 occupancy decreases from ~85% at 100 °C to ~70% at 200 °C at hydrothermal conditions.