The stability of the different minerals that constitute some ultrabasic massifs of Rio Grande do Sul, submitted to the weathering under the subtropical climate, is analyzed according to the composition of the underground waters. For this analysis, physico-chemical balance calculations are made between minerals and their hydrolysis solutions, as well as between primary minerals and those produced by weathering. These calculations make it possible to draw lines corresponding to the equations of each reaction and to delimit stability fields in graphs whose coordinates are the concentrations of Mg ++ and H 4 SiO 4. In these graphs, the average compositions of the subsoil waters are released per season, which indicate the scale of relative stabilities of the different minerals and the mineralogical phases that are in equilibrium with the environmental conditions.
