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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.
The geological formation surrounding a nuclear fuel waste disposal vault is an important barrier to the migration of radionuclides to the biosphere. Field investigations of plutonic rocks in the Canadian Shield have shown that open, water-bearing fractures form the main potential migration paths for radionuclides through the geosphere. Any interaction between the radionuclides in solution and the minerals in these fractures will retard radionuclide movement towards the biosphere. The minerals lining these open fractures are products of rock alteration and have themselves been subject to varying degrees of alteration over time. Accordingly, the fracture mineralogy is quite different from the bulk mineralogy of the intact host rock.
The degree to which measured radionuclide sorption varied with mineralogy was examined by laboratory sorption studies using static batch techniques and autoradiography combined with petrography. Fission products (Sr, Cs) and actinides (U, Np, Pu) were among the radionuclides studied. It was found that most fracture-infilling minerals such as hematite, goethite, illite, kaolinite, chlorite, epidote, gypsum and muscovite show preferential sorption for certain radionuclides. Calcite sorbs actinides such as U, Np and Pu from solution, but not ion exchangeable radionuclides such as Cs and Sr. Quartz shows the least affinity for any of the radionuclides studied.
It is concluded that variations in the type, amount and composition of fracture-infilling minerals must be considered in the assessment of the suitability of any geological formation as a radioactive wastes disposal site.
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