The < 2 mu (clay-size) fraction of a deep-sea sediment sample from the western North Atlantic Ocean was analyzed at varied concentrations for montmorillonite, illite, kaolinite and chlorite by X-ray diffraction of samples on silver filters. The results indicate that samples with concentrations on the filters of less than about 1.6 mg/cm 2 will show an artificial depletion of montmorillonite or mixed-layer montmorillonite-illite and enrichment of illite relative to samples of higher concentrations. Calculated compositions of kaolinite and chlorite tend to remain constant at any concentration where these minerals are readily detectable. The implications of this experiment are important for mineralogical studies of suspended particulate matter samples from the deep ocean, where samples with a total weight of only a few milligrams are normally recovered. X-ray diffraction analysis of such small samples may give the spurious results of montmorillonite-poor, illite-enriched particulate matter.