Formation and hydration enthalpies of the hydrosodalite family of materials

The four end-member structures of the hydrosodalite family of materials (Na6+x(OH)x[Al6Si6O24](H2O)N) are synthesized via hydrothermal techniques and characterized using X-ray diffraction, thermal analysis, scanning electron microscopy, and inductively coupled plasma optical emission spectroscopy. High-temperature drop-solution calorimetry in molten 2PbO · B2O3 at 975 K is used to measure the formation and hydration enthalpies. We report the formation enthalpies, from both the oxides and the elements, for a total of eight samples, two for each end-member. The average formation enthalpy (from oxides) is −88.1 ± 1.0 kJ/mol-TO2 for basic sodalite {ideally Na8(OH)2[Al6Si6O24](H2O)4}, −79.6 ± 1.2 kJ/mol-TO2 for hydroxysodalite {ideally Na8(OH)2[Al6Si6O24]}, −68.0 ± 1.1 kJ/mol-TO2, for hydrosodalite {ideally Na6[Al6Si6O24](H2O)8}, and −54.9 ± 1.1 kJ/mol-TO2, for sodalite {ideally Na6[Al6Si6O24]}. The corresponding average hydration enthalpies are −76.7 ± 5.3 kJ/mol-H2O and −37.0 ± 2.4 kJ/mol-H2O for the Na8 and Na6 series, respectively.