An experimental study of the phase equilibria in the system ZnO-SiO 2 -H 2 O has been made at elevated temperatures in the range 130 degrees C to 780 degrees C employing water vapor pressures from 500 to 40,000 pounds per square inch. Stability regions were established for several minerals in this system, which occur naturally in the oxidized zones of lead and zinc deposits. Hemimorphite was found to be stable up to about 250 degrees C at 20,000 psi., while above this temperature, willemite plus water are in equilibrium. With suitable qualifications this temperature may be taken as the maximum temperature of formation and stability of hemimorphite as well as the minimum for willemite. The pure zinc oxide-silica end member of the montmorillonoid-type mineral, sauconite, was found to be stable up to 210 degrees C, although natural sauconite, containing alumina is stable to approximately 125 degrees C higher. Uncertainties still remain in the determination of the Zn(OH) 2 <--> ZnO + H 2 O equilibrium, and for that reason studies of the reaction ZnCO 3 <--> ZnO + CO 2 may prove to be more significant in determining the minimum temperature of zincite formation.