Leonhardite, a partially dehydrated laumontite, and its alkali variety, primaryleonhardite, 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 Ca1.3Na0.6K0.8Al4Si8O24.7H2O. 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 K2O3 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 (K3Na) onenergetics 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.

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