Abstract: Dioctahedral tosudite, a regular interstratification of dioctahedral chlorite-dioctahedral smectite, occurs associated with kaolin in the hydrothermal area of Delgado, Neutla, Mexico. Its composition corresponds to the formula:

(Si_13.77Al_2.23)Al₈O₄₀(OH)₈·Ca_0.39Na_0.03K_1.06·(Al_3.79Fe⁺³_0.09Fe⁺²_0.06Mg_0.26)(OH)₁₂.

It forms as thin irregular flakes up to 5 µm in size. Adsorbed and cation hydration interlayer H₂O is lost at 81°C and 184°C dehydroxylation is intense at 496°C and weak at 656°C, with recrystallization at 970°C and 989°C. Infrared analysis shows OH-stretching at 3605 cm⁻¹ assigned to the Al-OH-Al group and at 3628, 3500, and 3365 cm⁻¹. Also, OH-bending occurs at 822 cm⁻¹, deformation of the H₂O molecule at 1630 cm⁻¹, Si-O stretching at 1020 cm⁻¹, and bending at 482 cm⁻¹, displaced by Al substitution and increase of the Si-O distance. The characteristic basal spacing of 29.49 Å for the air-dry mineral is changed to 31.32 Å when solvated and to 23.23 Å upon heating; d₀₆₀ = 1.496 Å. The interstratification is a regular 1:1 dioctahedral chlorite-dioctahedral smectite, R = 1, with coefficient of variability 0.73% for the air-dried mineral and 0.76% for the solvated one.