Electron microprobe analyses are made together with X-ray-diffraction determinations of diagenetic mixed-layer clays, which have an R3-type structure and are found in two Jurassic sandstone units of the North Sea Viking graben. Higher and more variable smectite content (2-12%) of more deeply buried layers is attributed to differences in pressure or fluid composition during crystallization (diagenesis). Neither temperature nor whole-rock composition seems to have been responsible for the differences in mineralogy found. It is necessary to consider not only the possible effects of temperature on clay-mineral solid-solution series such as the illite/smectite mixed-layer clay but also those of pressure (lithostatic versus hydrostatic) and aqueous solution composition.

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