40 



UNITED STATES MINERAL RESOURCES 



dred square miles and were estimated to contain 

 800 million tons of this low-grade rock; however, 

 part of this theoretical resource of alumina has 

 been lost in mining. 



Research has been carried on by industry and gov- 

 ernment organizations on the possibility of recov- 

 ering alumina, uranium, and phosphate from these 

 deposits and similar ones overseas. Some of the 

 earlier efforts were sufficiently encouraging to gen- 

 erate the issuing of several U.S. and foreign patents, 

 but no profitable extraction of aluminum has been 

 achieved. 



ALUMINOUS SHALE AND SLATE 



Aluminous shale, containing 20-24 percent AI2O3, 

 and slate, some of which is considerably richer in 

 alumina, are widely distributed throughout the 

 United States. The most extensive aluminous shale 

 units were formed from fine-grained marine sedi- 

 ments. Those in Central and Eastern United States 

 are of Paleozoic age, and those in the High Plains 

 and Rocky Mountain regions are of Cretaceous age. 

 Most slates in Eastern United States occur in a belt 

 extending from Maine to North Carolina, and they 

 are of Paleozoic age. 



Shales have been investigated as possible sources 

 of aluminum by several State agencies and by in- 

 dustry. The North American Coal Co. built a pilot 

 plant at Powhatan Point, Ohio, and tested the shaly 

 roof stone above the Pittsburgh coal bed. This at- 

 tempt at commercial production was unsuccessful. 



DAWSON ITE 



Dawsonite, NaAl(OH)2C03, a colorless or white 

 acid-soluble mineral, occurs in small needlelike 

 crystalline aggregates or as fine-grained to micro- 

 scopic crystals in several types of host rocks. Daw- 

 sonite contains, by weight, 35.4 percent AI2O3 or 

 18.7 percent aluminum metal. When heated it loses 

 hydroxyl water and carbon dioxide and forms water- 

 soluble sodium aluminate. Until its discovery in the 

 Tertiary oil-shale deposits of Colorado, dawsonite 

 was a mineralogical curiosity found only at a few 

 localities in the world (Smith and Milton, 1966; 

 Loughnan and See, 1967). 



Large quantities of dawsonite and nahcolite (see 

 discussion of nahcolite in chapter on "Evaporites 

 and Brines") are present in the rich oil-shale de- 

 posits of the Tertiary Green River Formation in the 

 Piceance Creek basin, northwestern Colorado. Daw- 

 sonite occurs as microscopic crystals disseminated 

 throughout the oil shale in sequences as much as 

 1,000 feet thick toward the center of the basin. The 

 top of the dawsonite- and nahcolite-bearing oil shale 



in the central part of the basin ranges from about 

 1,100 to 2,000 feet below the surface. The dawsonite 

 content of the oil shale ranges from a few percent 

 to at least 20 percent (Hite and Dyni, 1967). From 

 an examination of well cuttings, DeVoto, Stevens, 

 and Bloom (1970) identified a 500-foot-thick se- 

 quence of oil shale of potential commercial value in 

 a 250-square-mile area in the northern part of the 

 basin. The shale in this sequence contains a mini- 

 mum of 1.5 percent acid-extractable aluminum 

 (equivalent to 8 percent dawsonite), which amounts 

 to a resource of about 19 billion short tons of equiva- 

 lent dawsonite. In addition to the aluminum, the 

 rock contains an average of about 30 gallons of 

 shale oil per ton. 



Alumina for production of aluminum metal and 

 sodium aluminate for use in water-pollution control 

 are potential products of dawsonite in the Colorado 

 oil-shale deposits (Hite and Dyni, 1967; Irvin Neil- 

 sen, written commun., 1972). Recovery of either 

 product seems most feasible after the rock has been 

 retorted for shale oil. During retorting, the daw- 

 sonite would be converted to sodium aluminate 

 which could then be extracted by a water or caustic 

 leach (Dyni, 1972; VanNordstrand, 1968). Alumi- 

 num hydroxide could be precipitated from the 

 sodium aluminate solution, then calcined to form 

 alumina. Additions of nahcolite to the oil shale 

 before retorting might improve the recovery of 

 sodium aluminate (Hite, 1969). Although the 

 amount of extractable aluminum is very low to 

 serve as a commercial source of aluminum, the costs 

 of mining and processing the rock would be partly 

 ofl'set by coproduction of shale oil and nahcolite (or 

 soda ash) from the same rock. Furthermore, extrac- 

 tion of sodium aluminate from retorted dawsonite- 

 bearing oil shale does not require the Bayer process 

 which is necessary in processing bauxite for alu- 

 minum metal. 



The dawsonite-bearing oil-shale deposits in Colo- 

 rado originated in an ancient closed-basin lake en- 

 vironment whose waters were rich in organic matter 

 and sodium carbonate. Vast amounts of carbonate 

 minerals including dawsonite, nahcolite, and dolo- 

 mite were precipitated from strongly alkaline solu- 

 tions by mechanisms that are as yet unclear. Daw- 

 i sonite seems to be restricted to the Piceance Creek 

 basin; it has been looked for but not found in the 

 Green River Formation in Utah or Wyoming. Else- 

 where in the world small amounts of dawsonite 

 have been found in dikes at Montreal, Canada, in 

 sandstone from the Permian Greta Coal Measures 

 in Australia, in alluvial claystones of Pleistocene 

 age in Olduvai Gorge, Tanzania, and in a few other 



