The thermodynamic properties of petalite (li2al2si8o20) were determined by the Bureau of Mines. The enthalpy of formation was determined by hydrofluoric acid solution calorimetry. The values from the elements and from the oxides are ah deg. 298.15F = 2,335.8 +/- 3.0 Kcal/mole, and ah deg. 298 (From oxides) = -50.60 +/- 1.20 Kcal/mole. Low-temperature heat capacities were determined by adiabatic calorimetry from 10.69 to 302.11 K. The derived standard entropy is s deg. 298.15 = 111.0 +/- 1 Cal/deg-mole. Enthalpy increments above 298 k were measured by copper block calorimetry from 298 k to 1,193.6 K. The various experimental data were combined with other data from the literature to calculate the Gibbs energies of formation and equilibrium constants of formation over the temperature range of the measurements. Tables of enthalpies of formation and Gibbs energies of formation are given as a function of temperature from the elements and constituent oxides. The Gibbs energy of formation is ag deg. 298.15F + 2,203.8 Kcal/mole.