THERMODYNAMIC PROPERTIES OF THE ZEOLITE STILBITE

In this study, calorimetric measurements of the molar enthalpy of formation ΔfH0m at 298.15 K, molar heat capacity C0p,m (and, thus, the molar entropy 7 S0m) from 5 to 350 K, and enthalpy increments H0m(T) - H0m (298.15 K) from 350 to 500 K were performed on a specimen of stilbite characterized by electron microprobe analysis. The empirical formula of the stilbite was Ca1.019Na0.136K0.006Al2.180Si6.820O18·7.33 H2O. The following thermodynamic properties of stilbite at T = 298.15 K were determined: ΔfH0m(T) = -11033.6 ± 6.6 kJ mol-1, C0p,m(T) = 808.73 ± 1.62 JK-1 mol-1, S0m(T) - S0m = 805.54 ± 1.61 JK-1 mol-1, ΔfS0m(T) = -2990.44 ± 1.86 JK-1 mol-1 and ΔfG0m(T) = -10142.0 ± 6.6 kJ mol-1. The present ΔfG0m (stilbite) was combined with thermodynamic quantities from the literature to determine an equilibrium temperature for the reaction: stilbite = heulandite + H2O that is in close, but not exact, agreement with experimental phase equilibria. Retrieval of ΔfG0m (laumontite) from calorimetric and experimental values for the reactions: stilbite = laumontite + 3 SiO2 + 3 H2O and heulandite = laumontite + 3 SiO2 + 2 H2O gave results of -6778.9 and -6765.4 kJ mol-1. © 1990 Butterworth-Heinemann.