Abstract: Studies of the Recent indicate that, at the most, somewhat less than half the clay minerals are altered to any extent in a marine environment. Probably most of this alteration is in the form of cation adsorption or reconstitution of slightly weathered illites and chlorites to their original form. There appears to be little if any evidence that the detrital basic lattice is being altered to a measurable extent in Recent marine sediments. In the near-shore environments there is usually a coincidence of clay mineral suites and environments.
A major change in the clay mineral composition of sedimentary rock occurs within the Mississippian. Illite is the dominant clay mineral of the pro-Upper Mississippian sediments. Post-Lower Mississippian clay suites are more variable in composition ; illite becomes less abundant and montmorillonite and kaolinite more abundant. This change is best related to a change in regional tectonics. The clay minerals seem to have no preferred lithologic associations, although owing to epigenetic alterations porous sandstones commonly have different clay mineral suites from those of adjacent shales and carbonate rocks.
In many instances clay mineral facies coincide with environmental facies. As the clay mineral criteria for distinguishing any given type environment are extremely variable, it is thought that segregation of clay mineral suites by sorting is usually more effective than by diagenesis.
Expanded clay minerals appear to be partially contracted by the time they have been buried to 10,000–15,000 ft. It remains to be proved whether this is caused by chemical modification of the basic lattice with burial or if the detrital clay lattice has the inherent ability to contract, without chemical rearrangement, when buried to a sufficient depth.