210 



UNITED STATES MINERAL RESOURCES 



sulfate and high in ammonia and total solids (White 

 and others, 1963, p. 9, tables 13, 16, 29). They come 

 from diverse geologic settings and areas, and their 

 origin is not clear. Lake brines with comparable 

 compositions occur in still other environments 

 (Smith, G. I., 1966b). 



Resources of calcium chloride in the world are 

 large and, for practical purposes, nearly inexhausti- 

 ble. Production in the United States is now limited 

 primarily to the number of wells drilled in known 

 favorable areas, but these and new wells could 

 almost certainly supply domestic needs for at least 

 100 years. Billions of tons of calcium and mag- 

 nesium chloride are available from tachyhydrite 

 bodies in Brazil and western Africa (Ivanov, 1969; 

 Hite, 1972; Wardlaw, 1972). This amount repre- 

 sents thousands of years supply at present world 

 consumption rates. 



MAGNESIUM COMPOUNDS 



Magnesium and its compounds are produced from 

 bedrock deposits, natural and synthetic brines, and 

 sea water. The principal minerals mined for their 

 magnesia (MgO) content are magnesite (MgCOa), 

 brucite (Mg(OH).), and dolomite (MgCa(C0,)2) ; 

 deposits of these minerals are discussed in "Mag- 

 nesian Refractories" and "Limestone and Dolomite." 

 Most of the domestic and world production consists 

 of magnesium metal or one of several grades of 

 magnesia, although some is in the form of mag- 

 nesium hydroxide, magnesium sulfate, magnesium 

 trisilicate, and magnesium chloride. Magnesium 

 metal is used extensively in aircraft manufacturing 

 and other purposes for which its light weight is a 

 factor, and magnesia is used chiefly as a refactory 

 for the steel and other basic industries (Comstock, 

 1963; Lewis and Cammarota, 1971; Paone, 1970). 



Many industrialized nations produce magnesium 

 and magnesia from the sea that borders them. In 

 the United States, magnesium and its compounds 

 were produced in 1970 from deep-well brines in 

 Michigan and from sea water in five States. The 

 deep-well brines in Michigan supplied most of pro- 

 duction. Facilities for extraction of magnesium com- 

 pounds from Great Salt Lake brines are being con- 

 structed. Extraction of magnesium from sea water 

 or brine also requires calcined limestone or dolomite, 

 the latter yielding magnesium in quantities about 

 equal to that from sea water. 



The geologic environments of the raw materials 

 used in this industry are extremely diverse. Dolo- 

 mite and magnesite occur widely and were deposited 

 in a variety of environments. Magnesium in usable 

 concentrations is found in many surface and sub- 



surface brines that are dominated by sulfate or 

 chloride anions and that are deficient in carbonate. 

 Evaporite deposits containing billions of tons of 

 tachyhydrite (2MgCl2-CaCU -121120) in Brazil and 

 western Africa represent possible magnesium re- 

 sources (Ivanov, 1969; Hite, 1972; Wardlaw, 1972). 

 For practical purposes, the U.S. and world resources 

 of magnesium and magnesium compounds in brines 

 and sea water are unlimited, and bedrock resources 

 are large. 



UNDISCOVERED RESOURCES OF EVAPORITES 

 AND BRINES 



MARINE EVAPORITES 



The ultimate supply of commodities derived from 

 marine evaporites is theoretically limited only by the 

 existing volume of such deposits. That volume is 

 huge. Deposits of marine evaporites tend to be 

 hundreds of feet thick, and Kozary, Dunlap, and 

 Humphrey (1968) showed that about 25 percent of 

 the world's continental area is underlain by one or 

 more of them. 



This known area of evaporites may be nearly all 

 the total area that exists. This surprising conclu- 

 sion stems from the fact that the area climatically 

 favorable for the formation of marine evaporites 

 at any given time in the earth's history is limited; 

 calculations based on these limits plus reasonable 

 geologic assumptions as to areas that might also by 

 physiographically favorable allow one to approxi- 

 mate the maximum area of evaporites that could 

 have formed since the close of Precambrian time. 

 One such calculation, described below, shows that 

 maximum area to be about 60 percent of the earth's 

 surface. However, after subtracting areas represent- 

 ing reasonable guesses of the area of nondeposition 

 and loss due to erosion, the remainder barely ac- 

 counts for the evaporite bodies now known. Less 

 conservative, but still reasonable, assumptions fail 

 to account for all known deposits. Alternative models 

 are virtually limited to those that increase the depo- 

 sitional area, and experimentation has shown that 

 geologically reasonable alternatives that make sub- 

 stantial differences in the conclusion are difficult to 

 find. For this reason, the conclusion regarding the 

 small number of undiscovered evaporite bodies 

 seems sound and not likely to be wrong by any 

 factor approaching an order of magnitude. A more 

 detailed explanation of calculating procedures and 

 assumptions follows. 



Virtually all marine evaporites apparently formed 

 where warm arid climates prevailed along coasts 

 or in large embayments connected with the sea. 



