The chemical and mineralogical analyses of the disaggregated material collected in the fracture planes of crystalline carbonate outcrops at three sites north of Trois-Rivières, Quebec, showed the formation of expanding phyllosilicates, smectite and vermiculite, mainly at the expense of the ferromagnesian minerals hornblende and augite. The clay minerals are enriched in Fe and Mg with respect to the parent material composition and are associated with the crystalline Fe oxides goethite and lepidocrocite. The moderate to low degree of mineral transformation is accounted for by the slow dissolution of the carbonates. Corrensite, a regularly interstratified clay mineral, was identified in 50% of the samples; it probably resulted from the hydrothermal alteration of phlogopite. Silicate alteration may have been responsible for a fraction of the swelling minerals found in the soils of the area.