The diagenesis of argillaceous rocks includes physical, chemical, and mineralogical changes occurring after the deposition of the sediment. Hedberg's observations, published in 1936, on the gravitational compaction of clays and shales serve well as a framework upon which to organize many of the subsequent researches on diagenesis of those rocks.

The preponderance of illite in marine mudstones (commonly shales) has been interpreted as a result of widespread diagenetic conversion of other clay minerals into illite. More extended researches indicate, however, that primarily formed, expanding montmorillonite is notably more resistant to conversion to illite than is the expanding clay originating as stripped or degraded illite. The conversion of clay minerals to illite or chlorite is probably a two-stage process: (1) cation exchange, controlled by energy relations, and (2) subsequent and more permanent cation fixation. Some glauconite is apparently formed diagenetically, although complete details of the process remain obscure. Clay minerals that are not readily susceptible to diagenetic change are indicators of provenance, whereas those modified by a change in surrounding conditions may indicate the environments of their deposition.

Diagenesis of non-marine mudstones may include fixation of potassium, and alternatively the depotassication of illite; kaolinization of other clays, desilication of clays to form hydrated aluminum oxides, and alternatively their silication; and conversion of montmorillonite to illite and glauconitic mica.

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