Artificial stacking faults can be created within a well-crystallized kaolinite by intercalating and removing hydrazine. X-ray powder patterns with electron microscopy show that the created defects are --b/3 translations with a proportion 0.30. The infrared spectrum of the treated kaolinite is not modifed with respect to the starting one. On the other hand, a natural kaolinite containing defects by displacement of A1 vacancies in a similar proportion shows an infrared spectrum significantly different from that of a well-crystallized kaolinite. The modification of the infrared spectra of natural disordered kaolinites is then related to the presence of defects by change of A1 vacancy positions INTRODUCTION