628 



UNITED STATES MINERAL RESOURCES 



natural rubbers to increase the resistance to heat 

 and abrasion, to decrease vulcanization time, and 

 to improve the aging properties of the elastomer. 



The remaining 10 percent of the tellurium 

 consumed in 1968 goes into many diverse but im- 

 portant uses. Tellurium is used as a catalyst in 

 oxidation, hydrogenation, dehydrogenation, chlor- 

 ination, dechlorination, hydroxylation, and phenol 

 condensation. Noteworthy properties of tellurium 

 catalysts are selectivity, good yield, and resistance 

 to poisoning. Tellurium has been used as a colorant 

 in ultramarine, black, blue, red, and brown glasses 

 and ceramics. Organic tellurides have been used as 

 antioxidants for lubricants, greases, hydraulic 

 fluids, and hydrocarbon fuels. Tellurium compounds 

 are used as antimicrobial and therapeutic agents. 

 Cyclotelluropentane-3, 5-diones, and various deriva- 

 tives are very powerful germicides. Tellurium has 

 been used in the treatment of leprosy with good 

 results. Tellurium dioxide suspensions are effective 

 in the treatment of seborrheic dermatitis of the 

 scalp. Tellurium is also used in blasting caps, bat- 

 teries, thermoelectric devices, solar cells, infrared 

 windows, and semiconductor devices. 



Selenium may be used as a substitute for tellu- 

 rium in metallurgy, ceramics, rubber, and thermo- 

 electric devices. Lead may be used to achieve ma- 

 chinability in steel and thus replace tellurium. 



CRUSTAL ABUNDANCE 



The crustal abundance of tellurium is unknown. 

 The element is so rare that analytical methods have 

 not been devised with sufficient sensitivity to deter- 

 mine tellurium in igneous rocks with adequate 

 reliability. Efforts have been made to estimate the 

 abundance of tellurium on the basis of selenium- 

 tellurium ratios in materials containing enough of 

 both elements to be measured, but this gives rise to 

 a second-generation error because the abundance 

 of selenium is arrived at similarly by use of sulfur- 

 selenium ratios. Recent estimates of the abundance 

 of tellurium are 0.001, 0.002, and O.OOX ppm (parts 

 per million). The selenium-tellurium ratio in copper 

 produced from porphyry copper deposits in the 

 United States is 1 to 0.4, or 2.5 to 1 ; the same ratio 

 for Sudbury ores of Canada is 15. Using 0.05 ppm 

 as the crustal abundance of selenium, the ratio of 

 2.5 gives 0.02 ppm tellurium for the U.S. porphyry 

 copper ores and 0.003 ppm tellurium for the 

 pyrrhotite-chalcopyrite-pentlandite ores of Sudbury. 

 This divergence in the selenium-tellurium ratios is 

 indicative of fundamental differences in the geo- 

 chemistry of these elements. 



GEOCHEMISTRY 



Sulfur, selenium, and tellurium belong to the same 

 family of elements; their physical and chemical 

 properties change progressively from the low atomic 

 weight sulfur through selenium to tellurium. An 

 example of how progressive change in physical 

 properties can affect dispersion of these elements 

 in volcanism is found in the boiling points of per- 

 tinent chemical forms. The boiling points of sulfur, 

 selenium and tellurium are 444°, 688°, and 1,390°C 

 respectively. Hydrogen sulfide is a stable gas at 

 ambient temperature; hydrogen selenide decom- 

 poses in moist air to give selenium and water; 

 hydrogen telluride is even less stable than hydrogen 

 selenide. Whereas sulfur dioxide is a relatively 

 stable gas at ambient temperatures, both selenium 

 and tellurium dioxides are solids that sublime at 

 317° and 450°C respectively. It is evident from 

 these differences in the three elements that sulfur 

 is very mobile and may be carried in the atmosphere 

 great distances. In contrast, tellurium will be found 

 close to the volcanic source. 



Differences in ionic radii materially alter the dis- 

 tribution of tellurium from the distribution of sul- 

 fur and selenium. The ionic radius of S~^ is 1.84 A, 

 of Se-= 1.98 A, and of Te-^ 2.21 A. Selenium can 

 readily substitute for sulfur in sulfide minerals but 

 the tellurium ion is too large to replace sulfur. 

 Consequently selenium occurs in sulfide minerals and 

 selenide minerals in isomorphous association with 

 sulfide minerals, whereas tellurium forms its own 

 minerals as tellurides. 



The minimum radius ratio 

 /radius of cation 



\ radius of anion 

 for stability of coordinated octahedrons with a lig- 

 ancy number of 6 (Pauling, 1960, p. 544-545) per- 

 mits the metals gold, silver, bismuth, mercury, 

 platinum, palladium, and lead to form stable crystal- 

 line tellurides, whereas tellurides of zinc, copper, 

 nickel, and iron are less stable. The metals gold- 

 lead listed above, with very large ionic radii, cannot 

 readily substitute in sulfides of zinc, copper, nickel, 

 and iron, nor can tellurium substitute for sulfur; 

 consequently, the rare metals gold, silver, bismuth, 

 mercury, platinum, and palladium tend to accumu- 

 late in residual ore fluids and to crystallize as tellu- 

 rides after most of the sulfide minerals have crystal- 

 lized. This sequence appears to occur in early mag- 

 matic sulfide deposits of the Sudbury (Canada) and 

 Merensky Reef (Africa) types in which platinum, 

 palladium, and bismuth tellurides are conspicuous, 

 as well as in the late-stage, low-temperature hydro- 



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