An activation energy of 19.6 ± 3.5 kcal/mole was found for conversion of synthetic beidellite with the composition Al2Si3.66Al0.34O10(OH)2K0.34 to mixed-layer illite/smectite. The size of this activation energy and the rate constants suggest that (1) the alteration of smectite to illite during diagenesis involves the breaking of chemical bonds in the 2:1 layers; (2) either an equilibrium or a kinetic interpretation for the range of mixed layering found in burial diagenetic sequences is compatible with the kinetic data; and (3) the formation of illite from smectite on the ocean floor will not be seen, even if the reaction is favored thermodynamically, because the reaction rate is too slow.

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