The form and thickness of the ferrihydrite coating on kaolinite basal surfaces was studied in mixes containing 1–10% ferrihydrite through a combination of TEM, shadowed replicas, selected area electron diffraction, and determinations of surface area and distribution of charge on the surfaces. At pH3 the ferrihydrite was found to attach initially as single particles to certain sites on the kaolinite. These particles grew laterally and vertically until with 4% ferrihydrite they were aggregates of from 8 to 25 layers of particles. With more ferrihydrite the aggregates coalesced. Despite the irregular cover on the kaolinite satisfactory electron diffraction patterns were obtained and as little as 2% ferrihydrite was detected in a mixture at pH3.