We present the results of experimental studies on mineral and phase transformations in the model system K2O(Li2O)-Al2O3-SiO2-H2O-HF at 300 to 600 °C and 100 MPa using the method of univariant assemblages. The phase diagrams involve equilibrium curves among topaz, andalusite, muscovite, pyrophyllite, AlF3, and KnAlF3+n built from our experiments, which have allowed us to determine the topaz stability field. Topaz is stable in solutions with HF concentrations from 3·10-3 to 8·10-1 m and with KF concentrations lower than 7.5·10-3 m. As temperature increases, topaz becomes stable at higher HF concentrations. Application of the results to the Akchatau greisen W-Mo deposit provides an explanation of the observed zonation as a manifestation of metasomatic processes and imposes constraints on the mechanism and conditions of formation of the Akchatau deposit as well as on the compositions of the F-rich fluids participating in the greisenization.