Crystalline and amorphous phases present in a sensitive clay from Oslo are examined using several independent analytical techniques. Major crystalline phases are feldspars (50 wt.%), hornblende (approx. 18 wt.%), quartz (8 wt.%), clay mica (12 wt.%) and chlorite (approx. 7 wt.%). Selective-dissolution studies in association with energy-dispersive X-ray investigations reveal polymictic assemblages that are cemented by amorphous iron oxide. Amorphous iron compounds (approx. 2 wt.%) also form discrete particles and coatings on hornblende. The dissolution technique disaggregates these cemented mineral assemblages and affords an improved definition of the constituent particles. It is inferred that the minute particles (0.2 microns) and the amorphous iron oxide which make up the polymictic assemblages probably are common constituents of Norwegian and Canadian clays. Although these materials can appear as one bonded mass, their roles in the formation of sensitive-clay microstructures are considered to be independent. By enhancing interparticle bonding at low stresses, the glacially derived colloidal particles may promote the development of high-porosity structures. The amorphous coatings, produced by in situ chemical weathering, act as cementing agents and augment the established soil structure. Variations in the degree of cementation exhibited by the engineering characteristics of clays reflect differences in timing of the cementation.