This report presents the results of a study of the clay mineral composition of a series of bottom core samples collected by the staff of the Scripps Institute of Oceanography of the University of California in Pacific Ocean off the California coast and in the Gulf of California. The clay minerals were studied by X-ray diffraction, differential thermal, optical, chemical, and electron microscopic methods following particle size fractionation by sedimentation and supercentrifuge procedures.

All of the samples from the Pacific Ocean and from the Gulf of California contained illite, montmorillonite, and kaolinite; generally illite was most abundant and kaolinite least abundant. A chloritic clay mineral could be identified definitely in some samples. The clay minerals were in very complex mixtures, including mixed crystallizations as well as mechanical mixtures of discrete phases. In general the crystallinity was lower, the individual size smaller, and the intergrowth more intimate than in ancient sediments which have been studied by the authors.

Small amounts of quartz were associated with the clay minerals in the 1 to 0.1 and minus 0.1 micron fractions. Small amounts of another nonclay mineral crystalline phase that is probably a feldspar were also found in the finest size fractions of many samples.

The analytical data suggest that kaolinite is slowly lost during diagenesis under marine conditions, perhaps being changed to an illite or chloritic clay mineral. The data also afford evidence that potash is taken up by the clay and suggest that it is taken up largely by partially degraded illite which is carried into the sea. Magnesium also appears to be taken up by the clay, perhaps by the illite.

The widespread occurrence of montmorillonite indicates that this clay mineral is not lost quickly if at all during diagenesis under marine conditions.

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