Layer-By-Layer Mechanism of Smectite Illitization and Application to a New Rate Law

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Abstract
A layer-by-layer mechanism explains important features of mixed-layer clay minerals formed during the illitization of smectite, including the occurrence of randomly interstratified illite/smectite, the transition to ordered interstratifications, and the development of long-range ordering. A variety of solid- state transformation mechanisms were tested with a stochastic model, which accounts for interactions among clay layers. The model produces most successful results when the reaction of smectite layers with one illite nearest neighbor is favored over smectites with no illite neighbors by a factor of about two, and over those with two illite neighbors by a factor often or more. Synthetic X-ray powder diffraction patterns calculated from model results compare well with those of iUite/smectite minerals. These results suggest a new kinetic rate law. Solutions to this rate law for reaction within sediments undergoing burial give mineralogical profiles with depth similar to those observed in subsiding sedimentary basins.