Abstract: A random-layer disordered kaolinite from Conception de Buenos Aires, Jalisco, Mexico, gives an X-ray diffraction pattern resembling that of halloysite but has a kaolinite morphology. The mineral was located in a lignite deposit and, apparently was formed by alteration of volcanic rocks from the Cenozoic, with rapid cooling and sedimentation in lacustrine deposits. It is associated with plagioclase, quartz, cristobalite, montmorillonite, goethite and organic material. The analysis of purified material (44.84 per cent SiO2, 40.36 per cent Al2O3, 0.30 per cent Fe2O3, 0.31 per cent CaO, and 13.99 per cent ignition loss) corresponds to the formula (Si3.88 Al0.11 Fe0.01) (Al3.99 Fe0.01)O10 (OH)8. X-ray diffraction indicated a d001 of 7.56 Å, an intense 020, and displaced and nonsymmetrical reflections corresponding to a bi- and tridimensional diffracting system closer to halloysite than to kaolinite. Computed cell edges were a = 5.18 Å, b = 8.97 Å, and c = 7.45 Å. Electron micrographs showed anhedral rounded particles averaging 0.22 µ in size, while infrared absorption indicated some displacement in the absorption bands. The relationships of the mineral to kaolinite, halloysite and disordered kaolinites are discussed in terms of the 7°38′ twist of the tetrahedra, the intensities of the reflections, and the calculated AlIV substitution.