Low-dip fractures in the Lac du Bonnet granite pluton, SE Manitoba, contain abundant illite. Petrographic examination showed that the illite has three modes of occurrence: (i) alteration product of biotite, (ii) microfracture (< 1 mm width) filling, (iii) macrofracture (> 1 mm) filling. Microprobe analyses of these three types of illites in a single sample gave distinct compositional characteristics that defined an evolutionary trend, depicting Al2O3 enrichment, on an Al2O3−(K2O + Na2O)−(MgO + FeO) triangular diagram. White micas in the rock matrix adjacent to illite-bearing fractures have higher K and Al contents than all the three types of illites, implying that they formed under different conditions. The sorption of four radionuclides, 137Cs, 147Pm, 241Am and 90Sr, under oxic conditions on illite-bearing polished sections was also investigated, using autoradiography combined with petrographic examination. Illite showed greater sorption compared with other minerals present in the polished sections.