Abstract
Two outcrops were studied which showed granite in the early stages of weathering. Reactions between specific mineral species were observed, and especially the formation of an illite-like phase which is similar to that of sedimentary illite. This phase has a composition distinct from that of the initial micas in the rocks, notably in its iron and magnesium content. This suggests solution-solid equilibria which involve ionic transfer beyond the grain bondary region where the mineral is found.