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Abstract--Dioctahedral tosudite, a regular interstratification of dioctahedral chlorite-dioctahedral smectire, occurs associated with kaolin in the hydrothermal area of Delgado, Neutla, Mexico. Its composition corresponds to the formula: (Si13.77A12.23)AlsO4o(OH)a "Cao.39Nao.oaKLo6" (A13 vgFe+ 3o ogFe + 2o o6Mgo.26)(OH)~2.It forms as thin irregular flakes up to 5 #m in size. Adsorbed and cation hydration interlayer H20 is lost at 81 ~ and 184~ dehydroxylation is intense at 496*(7 and weak at 656~ with recrystallization at 970~ and 989~ Infrared analysis shows OH-stretching at 3605 cm-~ assigned to the AI--OH-A1 group and at 3628, 3500, and 3365 cm -t. Also, OH-bending occurs at 822 cm -~, deformation of the H20 molecule at 1630 cm -~, Si-O stretching at 1020 cm -~, and bending at 482 cm -t, displaced by A1 substitution and increase of the Si--O distance. The characteristic basal spacing of 29.49 ,~ for the air-dry mineral is changed to 31.32 ,~ when solvated and to 23.23/~ upon heating; do6o = 1.496/~. The interstratification is a regular 1 : 1 dioctahedral chlorite-dioctahedral smectite, R = 1, with coefficient of variability 0.73% for the airdried mineral and 0.76% for the solvated one.

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