Diagenesis of illite in argillaceous sediments of the Rhinegraben

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Abstract
The diagenesis of illite from smectite was investigated in relation to temperature on marly shale core samples from different Tertiary stages (depths between 200 and 3000 m) by X-ray and chemical analyses. It is concluded that the alteration of smectite to illite primarily depends on the cumulative energy supply (temperature × time). Small grain size and high potassium concentrations in the pore solutions shorten the time necessary to complete the alteration. Potassium is supplied by decomposition of feldspar within the sediment so that the replacement of feldspar by secondary calcite increases the availability of potassium. Supply of potassium by pore solutions from distant sources is very unlikely due to reduced permeability in compacted shales. Illite diagenesis is influenced by the inherited layer charge of the initial smectite.