X-ray analyses of the clay minerals from thousands of sediments indicate that any of the major clay minerals can occur in abundance in any of the major depositional environments and there is no consistent coincidence between specific clay minerals and specific depositional environments. It is concluded that the great majority of clay minerals in sedimentary rocks are detrital in origin, strongly reflect the character of their source material, and are only slightly modified in their depositional environments.

The most common process acting on the clay minerals in marine environments is cation adsorption. The modifications produced by this process are secondary but because of the overemphasis of names rather than processes, they have been considered fundamental changes and the process has been called diagenesis. From the geologist’s viewpoint, the basic clay mineral lattice, which is inherited from the source material, is the most significant parameter of the clay minerals, and modifications caused by the adsorbed cations are secondary, derived parameters reflecting the character of the depositional environment. This concept of dualism is essential to the understanding of clay genesis and the significance of the two parameters must be understood before a genetic classification can be constructed and before clays can be used for geologic interpretation.

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