Mineral stability relations in the system Al 2 O 3 -SiO 2 -H 2 O were investigated via the aqueous silica-dependent equilibria. The results have applications to a wide variety of mineralizing environments and provide a basis for derivation of much needed thermodynamic data in the system.Paths of metasomatic alteration operating in natural systems can be outlined in terms of the log m SiO 2aq versus 1/T diagrams presented. Mineral dissolution in the Al 2 O 3 -SiO 2 -H 2 O system is nonstoichiometric (incongruent), with preferential loss of SiO 2 to the solution. Thus, prograde and retrograde silica-leaching and silica-fixation reactions, respectively, are the main chemical controls on the formation of assemblages in this system. These processes are coupled with the hydrolytic processes responsible for driving natural rock compositions into this restricted range, together with the common introduction of accessory components, notably sulfate, boron, and fluorine. Mineralizing environments range from low-temperature weathering and lateritization, through hydrothermal conditions involving strong hydrolytic leaching, to processes operating in the high-tempearture late magmatic realm. Laterites, solfataric precious metal deposits, porphyry coppers, massive sulfides, and greisen deposits are discussed, relating some important equilibria in the broader system containing MgO, CaO, and the alkalies as well.Equilibrium thermal dehydration curves for hydrous phases in the system were also derived using the above experimental approach. For 1 kb H 2 O these are: 273 degrees + or - 10 degrees C (kaolinite-pyrophyllite-quartz). 300 degrees + or - 10 degrees C (kaolinite-pyrophyllite-diaspore), 337 degrees + or - 10 degrees C (pyrophyllite-diaspore-andalusite), 394 degrees + or - 10 degrees C (diaspore-corundum), and 366 degrees + or - 10 degrees C (pyrophyllite-andalusite-quartz). No stable invariant points occur. Computed Gibbs free energies of formation are: pyrophyllite. -5.266.499; andalusite, -2,441.030; diaspore, -920.240; boehmite, -915.954; corundum, -1,579.124; kyanite, -2.442.609; and sillimanite, -2,438.569, as based on kaolinite value of -3,799.364 kJ/gf.