The mineral composition of the <2-μm (clay) and 2- to 20-μm (silt) size fractions of aerosol particles collected near the margins of the North Pacific Ocean was determined by X ray diffraction analysis. Although the same mineral phases are present in the two size fractions, there are significant differences in the quartz, plagioclase, and kaolinite concentrations. North Pacific aerosols are primarily composed of illite in both size fractions in addition to kaolinite in the <2-μm size class and quartz and plagioclase in the 2- to 20-μm size class. Quartz and plagioclase are present in significantly higher amounts in the 2- to 20-μm size fraction, while kaolinite is greater in the <2-μm size class. Western North Pacific aerosol particles contain significantly higher <2-μm quartz than samples collected in the east, and eastern North Pacific aerosols have significantly higher concentrations of <2-μm smectite than those from the west. ¿ American Geophysical Union 1994