Mineral reactions at grain contacts in early stages of granite weathering

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.