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Zeolites have been known since the mid-1750s, but prior to the early 1950s, most reported occurrences of zeolites were in fracture fillings and amygdules in igneous rocks, particularly basaltic lava flows. Indeed, most of the large attractive zeolite specimens in museum collections were obtained from lavas. In recent years, zeolites have been recognized as important rock-forming constituents in low-grade metamorphic rocks and in a variety of sedimentary rocks. Most zeolites in sedimentary rocks are finely crystalline, that is they occur as microscopic or submicroscopic crystals, and they are therefore of little appeal to mineral collectors; however, deposits of this type are voluminous and have great geologic significance and economic potential.
Zeolites are among the most common authigenic silicate minerals that occur in sedimentary rocks, and they form in sedimentary rocks of diverse lithology, age, and depositional environment. About twenty species of zeolites have been reported from sedimentary rocks, but only eight zeolites commonly make up the major part of zeolitic rocks. These are analcime, chabazite, clinoptilolite, erionite, heulandite, laumontite, mordenite, and phillipsite.
This chapter will consider chiefly the zeolites in sedimentary rocks, with emphasis on volcaniclastic deposits, which contain the largest concentrations of zeolites. The occurrence of zeolites in lava flows is mentioned only briefly. Journal articles on the occurrence and origin of natural zeolites have multiplied at a rapid rate since the Mineralogy and Geology of Natural Zeolites was first published in 1977 as Volume 4 of the Mineralogical Society of America’s Reviews in Mineralogy. The present review will highlight areas of more recent research on the occurrence and origin of zeolites and some of the coexisting minerals.
A wide variety of zeolites has been identified in sedimentary deposits, with the most common being clinoptilolite, analcime, heulandite, laumontite, and phillipsite. Less abundant zeolites include chabazite, erionite, mordenite, natrolite …