The thermodynamic properties of aegirine, of composition [(na0.869Fe3+0.7362Fe2+0.16Mn0.03Ca0.1121) (Al0.141Si1.9738) (O6)], were experimentally determined at the Bureau of Mines as part of its efforts to advance minerals technology. The standard enthalpy of formation was determined by hydrofluoric acid solution calorimetry. Low-temperature heat capacities were determined by adiabatic calorimetry from 19.1 to 304.6 K. The derived standard entropy was determined. The heat capacity was measured over the range 348 to 1,198 k by differential scanning calorimetry. These experimentally determined data were combined with data from the literature to calculate the Gibbs energies of formation and equilibrium constants of formation over the temperature range of the measurements. Standard enthalpies of formation are given as functions of temperature.