Transposed temperature drop calorimetry and hightemperature drop solution calorimetry in molten 2PbO·B2O3 at 977 K were used to study the energetics of some manganese oxides, namely pyrolusite (MnO2), bixbyite (Mn2O3), hausmannite (Mn3O4), and manganosite (MnO). The enthalpies of oxidation at 298 K in the manganeseoxygen system, which were determined by appropriate thermodynamic cycles, were (in kJ/mol of oxygen): –441.4 ± 5.8 for the reaction 6MnO + O2→ 2Mn3O4, –201.8 ± 8.7 for the reaction 4Mn3O4+ O2→ 6Mn2O3, and –162.1 + 7.2 for the reaction 2Mn2O3+ O2→ 4MnO2. These values agreed very well with previous data that were obtained using equilibrium measurements that were reported in the literature. Thus, direct calorimetric measurements were well suited to obtain reliable enthalpy of formation data for oxides that contain manganese in the 2+, 3+, and 4+ states. Using these new values of enthalpies and reliable standard entropies, the phase‐stability diagram of the manganeseoxygen system was constructed.

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The energetics of a set of natural and synthetic manganese dioxides having a framework or a layer structure were determined by high-temperature solution calorimetry using sodium molybdate as a solvent. Enthalpies of formation of the metastable manganese dioxides were calculated from the measured values of the enthalpies of drop solution. The stability of the open structure depends mostly on the nature of and the amount of the tunnel (or interlayer) cation and on the degree of hydration. It depends less strongly on the topology of the structure itself and on the average oxidation state of manganese. The synthesis of these metastable microporous materials is not limited by the energetics of the structure.