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Evidence has been presented from the northern Gulf of Mexico which suggests that, over periods of several million years, the smectite-illite transition is largely controlled by temperature. Although kinetics obviously control rates over shorter intervals of time, the progression from smectite to illite with depth can be viewed largely as a succession of equilibrium states provided source and hence composition have been relatively constant. The abruptness of the transition zone over which this reaction occurs is proportional to the product of reaction enthalpy and geothermal gradient. Arrhenius plots of the log of the equilibrium coefficient versus reciprocal of the absolute temperature yield reaction enthalpies ranging from 26,000 cal/mole along the south Texas coast to a low as 1,800 cal/mole in the Mississippi Delta. It has been found that clays with high reaction enthalpies are typically montmorillonites, derived from volcanic ash, with lattice substitution principally in the octahedral layers. However, low enthalpy smectites found farther east are derived from the Mississippi River provenance and are beidellites with predominant tetrahedral substitution and hence more mica-like in structure.
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