Zeolites are among the most common authigenic silicate minerals in sedimentary rocks. They are particularly abundant in deposits of saline, alkaline nonmarine environments. Zeolites, alkalic feldspars, or searlesite almost invariably replace tuffs of saline, alkaline lakes, and they commonly form a substantial part of the nontuffaceous pelitic rocks. Zeolites most commonly occur in marine and fresh-water deposits as alteration products of volcanic glass, and the largest volumes of zeolites occur in thick accumulations of tuffaceous sedimentary rocks. Zeolites have been found in many marine and fresh-water lithic and feldspathic sandstones and in a wide variety of chemical and biogenic sedimentary rocks: limestone, phosphorite, coal, bauxite, and ironstone. The thickest and most widespread monomineralic sedimentary zeolite deposits known are Mesozoic analcimolites of probable saline-lacustrine origin.
Several examples of zeolites in sedimentary rocks are briefly described to illustrate different modes of zeolite occurrence and document several aspects of zeolite origin. Zeolites that formed in deposits of saline, alkaline nonmarine environments are included in late Pleistocene and Recent tuffs of Teels Marsh, Nevada; late Pleistocene and Recent sediments of Owens, China, and Searles lakes, California; Recent sediments and Pleistocene Peninj Beds of Lake Natron, Tanzania; Quaternary deposits of Olduvai Gorge, Tanzania; Miocene beds of Kramer, California; the Green River Formation of Wyoming; and the Lockatong Formation of New Jersey. New data are presented for all of these examples except the Lockatong Formation. Examples that typify the zeolites of marine and fresh-water deposits include Recent ash deposits in the Gulf of Naples, palagonitic deposits of the Pacific Ocean, the John Day Formation of Oregon, Green Tuff formations of Honshu, and Triassic rocks of New Zealand.
Zeolite mineralogy correlates to a variable degree with composition of host rock, water chemistry of depositional and postdepositional environments, age, and burial depth. A direct relationship exists between silica content of zeolites and that of vitric tuffs deposited in marine and fresh-water environments; relatively low-silica zeolites are formed from vitric tuffs of saline, alkaline lakes, regardless of the silica content of the glass. Grain size and permeability seem to correlate with zeolite mineralogy in some tuffs. Both a high pH and high salinity favor rapid reaction of volcanic glass to form zeolites. Zeolitic mineral assemblages in thick sequences of sedimentary rocks are zoned from the most hydrous nearest the surface to the least hydrous at depth. The degree of zonation and the thickness of the zones varies considerably in different areas. The ratio of laumontite to analcime, heulandite, clinoptilolite, mordenite, and phillipsite generally increases as a function of burial depth. The ratio of analcime to clinoptilolite, mordenite, erionite, and phillipsite generally increases with age in rocks buried only to relatively shallow depths. The ratio of authigenic alkalic feldspars to alkalic zeolites increases with increasing age, salinity, and burial depth.
Zeolites may be precipitated chemically and they may form by reaction of volcanic glass and aluminosilicate minerals in aqueous solutions. Analcime is apparently a chemical precipitate in some saline, alkaline lakes, and extensive deposits of analcimic argillite and analcimolite m ay possibly form in this way. Volcanic glass reacts in aqueous solutions to form zealots in many sedimentary deposits, regardless of their environment of deposition. Reaction rates of glass range from a few hundred years to tens of millions of years, depending on composition of the glass and nature of the chemical environment. Hydrolysis of glass to form a phyllosilicate mineral is an important factor in providing the chemical environment suitable to the formation of zeolites in many marine and fresh-water tuffs. Quartz, plagioclase, pyroxene, leucite, nepheline, and montmorillonite are among the primary minerals which react to form zeolites in sedimentary deposits. Laumontite, albite, and K feldspar can be form ed by reaction of early formed zeolites in response to increasing burial depth, and early formed zeolites may react at shallow depths to form an analcime, albite, and K feldspar as a function of either age or salinity. Geologic evidence suggests that alkalic feldspars may possibly be stable relative to zeolites in quartz-bearing sedimentary rocks under most conditions of sedimentary burial. Many zeolitic reactions involve a substantial amount of metasomatism, most of which can be attributed to the transfer of constituents in aqueous solutions within a sedimentary bed or sequence of beds.