year of publication
Calorimetric measurements have been carried out on specimens of natrolite, mesolite, and scolecite in order to assess the thermodynamic properties of a zeolite series having the same alumino-silicate framework. Low-temperature heat capacity and high-temperature enthalpy increments of natrolite and scolecite were measured by adiabatic and drop calorimetric techniques. Standard enthalpies of formation at 298.15 K of all three zeolites were determined by solution calorimetry. Thermodynamic functions have been calculated for natrolite to 660 K and for scolecite to 470 K. The heat capacity and standard entropy at 298.15 K have been estimated for mesolite. The following values at T = 298.15 K and a pressure of 1 bar have been obtained:
With reference to zeolitic water, it is concluded that the entropy of water molecules in both natrolite and scolecite is similar to that in ice. A consideration of the X-ray results for the present specimen of natrolite leads to the conclusion that a contribution of about 9.2 J/(mol·K) to the standard entropy is made by the Al-Si disorder in the alumino-silicate framework. The new results allow calculations to be made by which the pressure and temperature effect can be deduced for such equilibria as:
natrolite + SiO2(aq) = 2 analcime
natrolite + Ca2+(aq) + H2O = mesolite + 2Na+(aq).
Results are consistent with observed parageneses in natural occurrences.
Work performed under the auspices of the Division of Engineering, Mathematical and Geosciences, Office of Basic Energy Sciences, of the U.S. Department of Energy.