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Tosudite, a regularly interstratified chlorite-smectite, crystallizes as an alteration mineral of several preexisting Al-bearing silicates (feldspars, kaolin minerals, chlorites) present in arkosic sandstones hosted in uranium deposits in Niger. X-ray diffraction patterns show a sharp superstructure at 29-29.6 Å for an air-dried state and a peak at 30.8-31.6 Å following ethylene glycol solvation. The 060 reflection at 1.507-1.509 Å indicates an overall dioctahedral character, and the very low coefficient of variation of the d00l reflections for the solvated mineral (0.03-0.13) permits validation of the regular interstratification justifying its identification as tosudite. Microprobe analysis allowed specification of the component layers of this mixed-layer mineral. The chlorite is a di-trioctahedral type analogous to sudoite (Si3Al4Mg2(OH)8), and the smectite component is a low-charge montmorillonite type (Si4Al1.67Mg0.33M+0.33(OH)4). Tosudite is characterized by large Al2O3 and MgO contents and small Fe content; its composition corresponds approximately to the formula ((Si7Al)O20(Al4.5Mg2.3Fe3+0.2)M+0.3(OH)10), where octahedral occupancy is ∼7. Scanning electron microscope (SEM) observations show that tosudite is closely associated with some uranium minerals: tosudite crystallization occurred during a late alteration event which post-dates burial diagenesis and during which uranium was remobilized by Mg-rich oxidizing fluids. [ABSTRACT FROM AUTHOR]