The “sand” fraction of a sample of marine shale (Danian, California) consists almost entirely of nearly uniform spheres of the zeolite clinoptilolite. Individual spheres are polycry-stalline. It is suggested that the zeolite formed diagenetically by alteration of matrix mont-morillonite in the presence of silica-rich pore waters. A suitably reactive silica source formerly was present; badly corroded remnants of diatoms and radiolarians persist.

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