EVAPORITES AND BRINES 



205 



source of salt in the United States. About half of 

 the production is from salt domes, and most of the 

 balance from stratified marine evaporites. Boreholes 

 show that stratified halite deposits occur in most 

 major evaporite basins and a few minor basins that 

 are distributed through 24 of the 50 States. The 

 deposits are typically extensive and thick, and tend 

 to be superposed one above the other over wide areas 

 (fig. 24 and table 42) . Few domes or beds are pure 

 salt, but their salt content may exceed 98 percent; 

 dolomite, shale, anhydrite, and other evaporites are 

 usually found as impurities. Individual beds in strati- 

 field rock salt range in thickness from a fraction of 

 an inch to several hundred feet, and they are both 

 massive and laminated. Salt domes have very com- 

 plex internal structures. 



Salt is also produced in the United States from salt 

 lakes in the arid Western States and by solar evapo- 

 ration of sea water. Unpurified salt scraped from dry 

 lakes normally contains small amounts of other com- 

 ponents that limit its use or require purification. 

 Nearly pure salt is produced by solar evaporation of 

 lake brines and sea water, but the process requires 

 an arid climate and large fiat areas for evaporation 

 ponds (Ver Planck, 1958) . 



Domestic resources of salt are virtually unlimited 

 (Lefond, 1969). Huge bedrock resources are known 

 in the Northeastern, Southeastern, and Central 

 Western States (fig. 24) ; salt lakes and solar evapo- 

 rated sea salt are close to populated areas of the 

 west coast states. 



Salt resources of the rest of the world are also 

 virtually unlimited. About 15 percent of the earth's 

 continental areas are underlain by beds of marine 

 salt (Kozary and others, 1968) that can be extracted 

 by excavation or solution mining as the need arises. 

 Even greater quantities could be extracted from sea 

 water. This conclusion does not mean that salt will 

 be extracted from sursurface layers or adjacent seas 

 by all countries that have these resources, because 

 the economics of producing and transporting the 

 product will remain dominant. What it does mean is 

 that requirements can be met if necessary for even 

 remotely foreseeable future demands. 



GYPSUM AND ANHYDRITE 



Gypsum (CaS0,-2H,0) and anhydrite (CaSO^) 

 are common evaporite minerals widely used in the 

 construction industry and agriculture. Gypsum, by 

 far the more important of the two, is used to retard 

 setting time in portland cement and as an agricul- 

 tural soil conditioner and fertilizer. In addition, gyp- 

 sum is calcined for use as plaster for construction 

 and industrial purposes, and for manufacturing wall- 



board and other prefabrication products widely used 

 in the building industries. Anhydrite is interchange- 

 able to a limited extent with gypsum for use as a soil 

 conditioner. In Europe and elsewhere, anhydrite has 

 had moderate use in manufacturing sulfuric acid and 

 ammonium sulfate. 



For more than a decade, the United States has pro- 

 duced annually about 25 percent of the world gypsum 

 supply, and at the same time it has been dependent 

 on foreign sources for about one-third of its gypsum 

 supply. Domestic production for the past 10 years 

 has ranged between 9.4 and 10.7 million tons per 

 year, with its annual value ranging mostly between 

 $35 and $40 million. Imports, chiefly from Canada, 

 Jamaica, and Mexico, have averaged almost 5.6 mil- 

 lion tons per year (Ashizawa, 1971). Approximately 

 50 percent of the domestic gypsum supply comes 

 from mines and quarries in Michigan, Iowa, Texas, 

 and California ; the remainder is from mining opera- 

 tions in 17 States. Both the domestic production and 

 the imports of gypsum are very sensitive to the 

 demands of the construction industry, and they 

 respond rapidly to any change in the activity of this 

 industry (Schroeder, 1970) . 



Both gypsum and anhydrite occur widely and 

 abundantly in virtually all marine evaporite basins 

 (Withington, 1962). Gypsum noiTnally predominates 

 over anhydrite at or near the surface and grades into 

 anhydrite at depths ranging from a few feet to a few 

 hundred feet. Geological evidence indicates that some 

 near-surface gypsum was formed by hydration of 

 primary or secondary anhydrite where meteoric 

 ground and surface water came in contact with an- 

 hydrite. The depth of complete hydration is im- 

 portant in exploitation because the presence of more 

 than a few percent anhydrite renders gypsum unfit 

 for many purposes. 



Some gypsum is mined from deposits formed in 

 continental basins by the evaporation of nonmarine 

 waters. Most of these deposits probably originated as 

 gypsum, and their lack of deep burial has kept anhy- 

 drite from forming. Most nonmarine deposits are 

 smaller than marine deposits but tend to be nearer 

 the surface. 



Domestic reserves and resources of gypsum are 

 plentiful but are unevenly distributed. They are con- 

 centrated in the eastern Great Lakes region, the cen- 

 tral midcontinent region, the Rocky Mountain belt 

 from Canada to Mexico, and the southern California 

 and Nevada region. Like salt, beds of gypsum also 

 underlie an appreciable part of the earth's conti- 

 nents ; but unlike salt, they cannot be mined by solu- 

 tion and must therefore lie near the surface to be 

 extracted at low cost. Nevertheless, available quanti- 



