Zeolite minerals, especially analcime and clinoptilolite, are known to occur widely in sedimentary rocks both as nearly monomineralic beds and as accessory minerals. Because of the extremely fine grain size of the zeolite rocks, identifications are made most readily with X ray diffraction techniques. A survey of the known occurrences of sedimentary zeolites shows that most, if not all, of these minerals were formed during diagenesis by the alteration of volcanic material. Rhyolite glass from vitric ash falls may be altered during diagenesis to clay minerals, zeolites, or authigenic feldspars; the mineralogy of the diagenetic products is a promising indicator of post-depositional chemical environments.

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