A mineralogical investigation of the less-than-2-μ fraction of 52 North Atlantic–Arctic Ocean sediment-core samples demonstrated that the constituent clay minerals are derived primarily from terrestrial sources and are transported to environments of deposition primaily by ice rafting and ocean currents. Clay-mineral compositions of marine sediments and clay-mineral compositions of soils of surrounding land masses vary in a similar way with respect to latitude. Pure illitic, chloritic, and montmorillonitic phases are apparently not stable in the marine environment but will diagenetically change to mixed-layer, expandable phases. Mixed-layer, expandable phases derived from pure illitic and chloritic phases are similar to, and generally indistinguishable from, mixed-layer phases formed from montmorillonitic phases or formed by regrading stripped chlorites and illites. Clay minerals cannot be used to determine paleoclimatic horizons in North Atlantic–Arctic Ocean sediments. They may be useful in a limited way to indicate weathering intensities in sediment source areas.