The diagenetic transformation of smectite to illite, with an intermediate series of mixed-layered compositions, has been documented by numerous studies of surface and subsurface rocks. Shales from typical Gulf Coast wells show compositions greater than 50% illite at approximately 100°C; the composition stabilizes at about 80% illite at the greatest depths sampled where the temperature is or has been as high as 130°C.

These findings are difficult to reconcile with data from oil (lower Paleozoic) rocks if it is assumed that illite-smectite compositions are equilibrium phases at specific temperatures. Potash bentonites (Ordovician)

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associated with thin cratonic sediments contain 80% illite in the mixed-layered species, and these rocks were probably never heated over 60°C. In addition, many shales and limestones of Devonian age and older contain almost pure illite despite histories of only moderate burial depths.

Studies of shales near contacts with basaltic dikes show that compositions near 80% illite are associated with peak temperatures (calculated) of approximately 300°C, though the duration of the heating event was short (^approx1,000 yr).

A synthesis of available information suggests that the smectite to illite transformation is kinetically controlled, and that a high-order rate law is required, despite laboratory synthesis results which fit a first-order kinetic scheme. These conclusions are not accepted by all investigators, and the details of the reaction stoichiometry, kinetic mechanisms, and the possible occurrence of equilibrium intermediate compositions have yet to be convincingly demonstrated.

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