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SUMMARY. Classical clay mineralogy determinations and electron microprobe analyses of weathering minerals developed in altered two-mica granites indicate that the chemical forces that produce new minerals are often constrained tosmall volumes, frequently on the scale of a mineral grain or contact between two grains in the granite. Chemical potentials uch as pH, alkali and alkaline earth and silica activity in the altering aqueous solutions provoke adestabilization f pre-existing minerals, which recrystallize locally to give a new multimineral product. The chemical composition of the new phases is largely governed by the relative concentrations of the elements present in the former minerals. Three mineral facies were observed in the weathere