Thermochemical study of calcium zeolites - heulandite and stilbite

Calorimetric measurements were made on natural samples of heulandite having the composition Ca(0.86)Na(0.37)K(0.06)Al(2.14)Si(6.86)O(18)6.1H(2)O and s tilbite Ca(101)Na(012)Al(2.12)Si(6.88)O(18)7.27H(2)O both from the same loc ality (Nidym River, E. Siberia. Russia). Enthalpies of formation and hydrat ion were studied by calorimetry in lead berate solvent at 975 K. The enthal pies of formation from oxides and elements at 298 K of heulandite are: -238 .7 +/- 4.9 kJ/mol, and -10656.3 +/- 8.6 kJ/mol, and of stilbite are -232.0 +/- 8.3 kJ/mol and -11017.9 +/- 10.9 kJ/mol respectively. The integral hydr ation enthalpies including the enthalpies of phase transitions during heula ndite and stilbite dehydration are -209.4 +/- 5.9 kJ/mol and -229.2 =/- 7.4 kJ/mol at 298 K, respectively. The molar Gibbs free energies of formation for idealized calcium-sodium and pure calcium heulandites and stilbites wer e calculated by combining these new calorimetric data with thermodynamic qu antities from the literature. Equilibrium temperatures for the reaction: Ca -stilbite = Ca-heulandite + H2O, calculated on the basis of these thermodyn amic data, agree with experimental phase equilibria.