Abstract: Transmission and analytical electron microscopy have been used to study the diagenesis of the trioctahedral component of phyllosilicates (principally chlorite) in argillaceous core samples (depths of 1750, 2450, and 5500 m) from the Gulf Coast. Chlorite was observed as 100–150-Å thick packets intergrown within mixed-layer illite/smectite in the 2450-m sample and was more abundant and larger in packet thickness in the 5500-m sample. The chlorite is disordered in stacking sequence as characterized by diffuseness of reflections with k ≠ 3n in electron diffraction patterns. Berthierine (7-Å) was locally found to be intercalated within chlorite layers. Chlorite from the 5500-m sample is iron-rich and has an average chemical formula of Fe3.1Mg1.1Al2.7Si2.8O10(OH)8. This chemical composition and textural relations suggest that the chlorite formed within mixed-layer illite/smectite utilizing Fe and Mg released from the smectite during its conversion to illite. The berthierine is nearly identical in chemical composition with the coexisting chlorite although it may have a slightly higher Fe content. The 7-Å berthierine is probably a metastable precursor of the chlorite and may be diagnostic of the diagenetic environment.