Abstract:
The thermochemistry of well-characterized synthetic K-H3O, Na-H3O and K-Na-H3O jarosites was investigated. These phases are solid solutions that obey Vegard's law. Electron probe microanalyses indicated lower alkali and iron contents than predicted from the theoretical end-member compositions, in agreement with thermal analyses, suggesting the presence of hydronium and ''additional'' water. The standard enthalpies of formation (ΔH°f) of K-H3O, Na-H3O and K-Na-H3O jarosites were determined by high-temperature oxide melt solution calorimetry. These enthalpies vary linearly with the K/H3O, Na/H3O and K/Na ratio, respectively. The enthalpy of formation of pure hydronium jarosite was also determined experimentally (ΔH°f = -3741.6 +/- 8.3 kJ.mol-1), and it was used to evaluate ΔH°f for the end-members KFe3(SO4)2(OH)6 (ΔH°f = -3829.6 +/- 8.3 kJ.mol-1) and NaFe3(SO4)2(OH)6 (ΔH°f = -3783.4 +/- 8.3 kJ.mol-1). Finally, enthalpies of dehydration (loss of the ''additional'' water) of some jarosites were determined and found to be near the enthalpy of vaporization of water, suggesting that the ''additional'' water is weakly bonded in the structure.
