This study investigated soil clay mineralogy of mangrove forests along the Brazilian coast in eight regions of different environmental characteristics, with a focus on the crystallochemical features and genesis of 2: 1 phyllosilicates. Samples from two different depths (0-30 and 60-90 cm) and two clay size fractions (<2 and <0.2 mu m) were studied. The analytical tools used were X-ray diffraction, Fourier-transform infrared spectroscopy, and transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS). A complex assemblage was found in both fractions, including high proportions of kaolinite, illite (Fe-illite/glauconite), and smectite (Fe-beidellite, Fe-montmorillonite), minor amounts of gibbsite and quartz, and traces of K-feldspar, halloysite, and amphibole crystals (TEM-EDS data). Evidence of illite-smectite and possibly kaolinite-smectite mixed-layer phases was found in the <0.2-mu m fraction. An authigenic transformation process likely taking place, from kaolinite to Fe-illite/glauconite, through transitory kaolinite- smectite and illite-smectite phases. The reaction is triggered by high Fe activity in solution generated by dissolution of Fe oxides and pyrite present in the sediment, at approximately neutral pH and high salt content in the water. Smectite illitization takes place by substitution of Al3+ for Si4+ in the tetrahedral sheet, of Fe and Mg2+ for Al3+ in the octahedral sheet, and progressive Fe reduction, all of which produces an increase in the layer charge and K uptake. The sequential nature of the transformation (kaolinite-smectite-illite) suggests a solid-state transformation process. Two unusual kaolinite-rich mangroves were found, indicating low reactivity or high deposition rate of continental sediments of soils derived from the Barreiras Group, dominated by kaolinite.