The Thermal Transformation of Smectite to Illite
Mixed-layered illite/smectite minerals composed of 80% illite layers have been identified from different argillaceous rocks that have been subjected to estimated peak temperatures that ranged from 250°C to 70°C, for durations of approximately 10 yr to 300 my. These observations strongly suggest that the reaction progress or extent is controlled by kinetic factors rather than by equilibrium factors. A sixth-order kinetic expression (first-order with respect to the pore-fluid activity ratio K/Na, and fifth-order with respect to the mole fraction of smectite) was successfully applied to the progressive illitization of smectite in the contact metamorphic zone adjacent to an 8.5-m-thick basalt dike that penetrates the upper Pierre Shale near Walsenberg, Colorado. The kinetic expression, together with its preexponential constant and activation energy (33 kcal/mol), provides a fair to good transformation model for a young geothermal sequence, and for burial diagenetic profiles that range in stratigraphic age from approximately a few million to 300 Ma.
The sixth-order model is an empirical device that explains the field evidence, but probably has little or no fundamental physical-chemical significance. The correct kinetic law is likely to be a chain of low-order reactions, each of which has kinetic constants that differ from the others.
KeywordsClay Mineral Gulf Coast Reaction Profile Contact Metamorphism Burial Diagenesis
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