UNITED STATES MINERAL RESOURCES 



ZEOLITES IN SEDIMENTARY ROCKS 



By Richard A. Sheppard 



CONTENTS 



Page 



Abstract of conclusions 689 



Introduction 689 



Exploitation 690 



Geologic environment 690 



Prospecting techniques 692 



Potential resources 693 



Research needed 693 



References 694 



TABLE 



148. Zeolites reported from sedimentary rocks 



ABSTRACT OF CONCLUSIONS 



Zeolites are among the most common authigenic silicate 

 minerals in sedimentary rocks; they occur in rocks that are 

 diverse in age, lithology, and depositional environment. 

 Zeolites are particularly common in Cenozoic sedimentary 

 rocks that originally contained abundant silicic vitric ma- 

 terial. Most zeolites in sedimentary rocks formed during 

 diagenesis by the reaction of volcanic glass with interstitial 

 water which originated as either meteoric water or connate 

 water of a saline, alkaline lake. Although about 20 different 

 zeolites have been reported from sedimentary rocks, only 

 analcime, chabazite, clinoptilolite, erionite, ferrierite, heu- 

 landite, laumontite, mordenite, and phillipsite commonly 

 make up the major part of zeolitic rocks. Nearly monominer- 

 alic beds of zeolites are known, but most zeolite rocks also 

 contain clay minerals, silica minerals, and feldspars of 

 authigenic origin. 



Production of natural zeolites in the United States is 

 currently only several hundred tons per year. The abundance 

 and variety of zeolite deposits in the United States, the 

 unique properties of the zeolites, and the seemingly low 

 cost of mining suggest potential uses in many industrial and 

 agricultural processes. Chemical and structural modifica- 

 tions of the natural materials could further increase their 

 potential. When new markets for zeolites are developed, the 

 United States will not only be self-sufficient but will prob- 

 ably become a major exporter. 



The total of identified, hypothetical, and speculative re- 

 sources of zeolites in the United States is conservatively 

 estimated at more than 10 trillion tons. Total resources of 

 zeolites in the rest of the world have not yet really begun to 

 to be cataloged. 



INTRODUCTION 



Zeolites — crystalline hydrated aluminosilicates of 

 the alkalis and alkaline earths — are important rock- 

 forming constituents in sedimentary rocks and are 

 potentially valuable industrial minerals. They have 

 a framework structure that encloses interconnected 

 cavities occupied by the relatively large cations and 

 water molecules. The cations, chiefly sodium, potas- 

 sium, and calcium, and the water have considerable 

 freedom of movement within the structure and give 

 the zeolite their cation-exchange and reversible- 

 dehydration properties. Some zeolites have essential 

 contents of barium, strontium, or magnesium. The 

 basic structure of all zeolites consists of (Si,Al)04 

 tetrahedra wherein each oxygen is shared between 

 two tetrahedra; thus, the atomic ratio 0:(Si-fAl) 

 is 2. The net negative charge of the structure is 

 balanced by the cations. The porous framework of 

 the zeolites enables them to act as molecular sieves 

 for the separation of molecular mixtures according 

 to the size and shape of the molecular compounds. 



More than 30 distinct species of zeolites occur in 

 nature. Numerous zeolites have also been synthe- 

 sized, but many of these have no natural counter- 

 parts. Most zeolites have a specific gravity of 2.0- 

 2.3 and indices of refraction of 1.44-1.52. Zeolites 

 rich in barium or strontium commonly have higher 

 specific gravities and indices of refraction. Because 

 the zeolites have similar physical properties, X-ray 

 diffraction techniques are generally used for identi- 

 fication. 



Synthetic zeolites have been produced commer- 

 cially in the United States since the early 1950's. 

 These synthetic zeolites are used principally as cata- 

 lysts, selective sorbents, and desiccants; they are 



U.S. GEOL. SURVEY PROF. PAPER 820 



