All the amides used can be intercalated to give single-layer complexes with basal spacings between 10.4 Aa (for formamide) and 12.4 Aa (for N, N-dimethylacetamide). When the interlayer space of halloysite has already been expanded by water, as is the case with fully hydrated samples, the rate and extent of intercalation depend more on the properties of the organic species than on the mineralogy of the clay. With partially and completely dehydrated halloysite, however, complex formation is influenced by the particle size, crystallinity, and iron content of the mineral. A large particle size, a high degree of crystallinity, and a low iron content are all conducive to complex formation. The iron content of halloysite also appears to affect particle shape. Low structural iron contents are associated with long tubular particles, intermediate contents with short and/or wide tubes, and high contents with spheroidal and other non-tubular forms. Halloysites containing high amounts of extractable (non-structural) iron generally show a low intercalating ability.