574 



UNITED STATES MINERAL RESOURCES 



desirable pink tinge to glass, and in still larger quan- 

 tities yields a ruby-red glass. Although the principal 

 use of selenium in flat glass is to decolorize, more 

 than 100,000 pounds of selenium per year is used to 

 produce "black" glass that is used as an outer sur- 

 face in many modem office buildings. 



The manufacture of inorganic pigments, principal- 

 ly cadmium sulfoselenide, accounts for about 14 per- 

 cent of the annual consumption of selenium. These 

 pigments have good light stability and heat resist- 

 ance and are used in plastics, paints, enamels, inks, 

 and rubber. 



The remainder of the selenium is consumed in 

 many diverse applications. For example, selenium 

 added to stainless steels improves their machining 

 characteristics. Selenium compounds are used as vul- 

 canizing agents in rubber products to promote resist- 

 ance to oxidation and abrasion, and in oils to obtain 

 exceptional antioxidant properties. Selenium and its 

 compounds are used in many catalytic reactions in 

 the preparation of drugs and hydrogenated oils. The 

 element also finds use in fungicides, bactericides, 

 parasiticides, herbicides, and insect repellents and in 

 photographic photosensitizers, phosphorescents, and 

 luminescents. A rather exhaustive listing of uses 

 may be found in the materials survey by Lansche 

 (1967). 



An anticipated very large demand for selenium re- 

 sults from the discovery in 1957 that selenium is 

 required in the nutrition of animals, including man. 

 In New Zealand, the addition of selenium to soils to 

 increase the selenium content of forage crops is not 

 only legal but widely done. The use of selenium in 

 fertilizers will probably be legalized in most coun- 

 tries in the near future. 



The adverse effects on the economy caused by 

 selenium deficiency are summarized by Oldfield 

 (1971) : 



Widespread field occurrence of white muscle disease in lambs 

 and calves, myopathy of the gizzard and heart in turkeys, 

 exudative diathesis in chicks, and liver necrosis in swine, 

 together with depression of growth and decreased efficiency 

 of feed utilization that result from selenium deficiency in 

 commercial feeds, represent heavy economic losses to poultry 

 and livestock producers in many part of the United States. 

 Selenium deficiency diseases with resultant economic 

 losses have also been reported in Canada, New Zea- 

 land, Australia, Scotland, Finland, Sweden, Den- 

 mark, France, Germany, Greece, Turkey, and Russia. 

 At dietary levels below 0.04 ppm (parts per million) 

 selenium, domestic animals suffer from selenium- 

 deficiency diseases ; dietary levels of 1 ppm selenium 

 are completely safe; dietary levels above 4 ppm 

 selenium cause domestic animals to suffer from 

 selenium-toxicity diseases. 



Selenium as sodium selenite can be added safely 

 and profitably at the rate of 1-2 pounds per acre once 

 every 5 years in areas where forage crops are de- 

 ficient in selenium. It is estimated that 12.5 million 

 pounds of selenium per year would be required to 

 treat the most severely selenium deficient areas of 

 the United States; this figure is based on a rate of 

 application of 1.5 pounds of selenium as sodium 

 selenite once every 5 years. To reach an optimum 

 nutritional level of selenium in forage crops in the 

 United States, 25 million pounds of selenium would 

 be needed annually. This consumption would be more 

 than 20 times the amount of selenium used in the 

 United States in 1968. 



More selenium would probably be used in industry, 

 especially in steel and glass manufacture, if the sup- 

 ply and price were stable. 



GEOCHEMICAL CYCLE 



Selenium and its compounds HaSe and SeOa are 

 sufficiently volatile to accompany sulfur in volcanic 

 gases. It has been estimated that approximately 2 

 pounds of selenium per square yard has been de- 

 posited on the earth's surface by volcanic gases. 

 Through this process, selenium tends to become a 

 dispersed element, but other geologic processes tend 

 to concentrate it. 



Selenium associated with intrusives accompanies 

 sulfur into sulfide minerals. The ionic radius of Se~- 

 is 1.98 A and that of S-= is 1.84 A; thus Se-= can 

 substitute for S~- in many sulfide minerals. The 

 high-temperature sulfides, pyrrhotite and pentland- 

 ite, have a sulfur-selenium ratio of 7,000, a value 

 near that given by Goldschmidt and Strock for the 

 crustal abundance estimate. In sulfide minerals from 

 the mines of the Fahlun area in northern Sweden, 

 the order of decreasing selenium content is galena, 

 chalcopyrite, arsenopyrite, sphalerite, pyrite, and 

 pyrrhotite. The order varies, however, in other areas. 

 In porphyry copper ore containing 0.5 percent cop- 

 per, the selenium content is 2.5 ppm. In these ores, 

 copper is enriched over its crustal abundance 100 

 times, but selenium is enriched only 50 times. 



In weathering, selenium and sulfur tend to be 

 separated, in large part because selenium becomes 

 fixed in insoluble basic ferric selenites. Only in an 

 alkaline oxidizing environment is selenium oxidized 

 to the soluble selenate form ; sulfur, however, is even 

 more readily oxidized to highly soluble sulfate and is 

 carried away in surface and ground waters. As a 

 result, selenium is present neither in sulfate deposits 

 nor in sedimentary sulfur deposits. 



In contrast to the separation of selenium and sul- 

 fur in weathering processes, the two elements travel 



