Thermochemistry of natural potassium sodium calcium leonhardite and its cation-exchanged forms

Leonhardite, a partially dehydrated laumontite, and its alkali variety, primary leonhardite, have been studied by high-temperature calorimetry. The enthalpies of formation from oxides and elements at 298 K are -306.7 ± 7.1 and -14214.6 ± 11.2 kJ/mol, respectively, for leonhardite, Ca₂Al₄Si₈O₂₄·7H₂O, and -521.2 ± 10.5 and -14253.7 ± 13.5 kJ/ mol, respectively, for primary leonhardite of composition Ca_1.3Na_0.6K_0.8Al₄Si ₈O₂₄·7H₂O. The values for primary leonhardite are significantly more negative. New calorimetric data for sodium and potassium oxides were obtained on the basis of thermochemical cycles involving carbonates. The enthalpies of drop solution are -113.10 ± 0.83 kJ/mol for Na₂O and -193.68 ± 1.10 kJ/mol for K₂O, giving enthalpies of solution of -170.78 ± 0.90 kJ/mol for Na₂O and -260.98 ± 1.20 kJ/mol for K₂O. The effects of exchange cations (K,Na) on energetics and dehydration were studied using cation-exchanged samples. Alkali substitution decreases thermal stability (decomposition on heating in air) but increases thermodynamic stability with respect to the oxides and elements. Equilibrium relations between leonhardite and alkali-feldspar, calculated on the basis of these data, show that primary leonhardite can form only from geothermal solutions having rather high ratios of alkali ions to Ca.