Palygorskite and sepiolite were digested with large excesses of hydrochloric acid at constant temperature for various periods of time. The reaction was found to be of first order with respect to the magnesium, aluminum and iron components in the clay and also with respect to hydrochloric acid concentration. The reaction rate constant for Mg is larger than for Fe, which is larger than for Al. Approximately one-third of the octahedral-type cations are extraneous, suggesting that only three of the five octahedral positions are filled. The reaction rate constant of Mg in sepiolite is about 240 times that for palygorskite. This may be an indication that sepiolite is less stable to chemical weathering than palygorskite. The activation energies for Mg, Al and Fe in palygorskite were found to be the same and equal to about 18·4 kcal per mole. Treatment of partially acid-digested palygorskite with magnesium ions under alkaline conditions resulted in an increase in the X-ray intensity of the 10·5 Å spacing, suggesting some of the palygorskite was reconstituted. © 1969.
