CADMIUM 



107 



Table 20. — Zinc and cadmium contents and zinc: cadmium, ratios in selected 

 geologic materials 



Geoloerlc material 



Average metal 

 content (ppm) 



Crustal abundance 



Do 



Igneous rocks 



Do 



Magmatic sulfides 



Swedish sphalerites 



Estimated average world zinc concentrate. 



Joplin sphalerites 



Southeastern U.S. sphalerites 



Permian phosphatic shale, Wyo 



Bentonites in Pierre Shale 



Black shale 



Kupferschiefer ore, Mansfeld district 



9,000 



500 



18 



Zn — "Crustal Abundance," this 

 volume; Cd— Parker, 1967. 



Tourtelot and others, 1964. 



Rankama and Sahama, 1950. 



Sandell and Goldich, cited by Rank- 

 ama and Sahama, 1950. 



Rankama and Sahama, 1950. 



Gabrielson, cited by Rankama and 

 Sahama, 1950. 



"Zinc," this volume. 



Siebenthal, 1909. 



Maher and Fagan, 1970. 



Love, 1961. 



Tourtelot and others, 1964. 



Davidson, cited by Tourtelot and 

 others, 1964. 



Cissarz and Moritz, cited by 

 Rankama and Sahama, 1950. 



analog of isometric sphalerite, and as greenockite, 

 the cadmium analog of hexagonal wurtzite — sphaler- 

 ite and wurtzite are the two crystalline varieties 

 of zinc sulfide (ZnS). The anhydrous carbonate 

 (CdCOs), otavite, is the cadmium analog of smith- 

 sonite (ZnCOs) and can occur as oriented over- 

 growths on zinc carbonate crystals. Otavite was first 

 found at Tsumeb, near Otavi, South West Africa. 

 The most common of the independent cadmium min- 

 erals is the sulfide, greenockite. It occurs chiefly as a 

 yellowish earthy film or coating on zinc minerals, 

 mainly sphalerite, and rarely as crystals. These coat- 

 ings form as a result of the low-temperature altera- 

 tion of sphalerite, chiefly alteration due to 

 weathering. 



ORES 



None of the independent cadmium minerals is 

 abundant enough to be an ore mineral, although the 

 presence of trace greenockite in slightly altered or 

 oxidized sphalerite will slightly enhance the cadmi- 

 um content of the zinc-bearing ores. The chief source 

 of commercial cadmium is zinc ore, and cadmium is 

 by far the major metallic byproduct recovered in the 

 smelting of zinc concentrate and the refining of zinc 

 metal. Thus, the major concentrations of cadmium 

 reasonably well known today are in the sulfide zinc 

 ores, although potential resources exist in other zinc 

 ores and in other lesser known potential zinc con- 

 centrations. (See chapter on "Zinc") 



The cadmium content of sphalerite is highly vari- 

 able. This zinc sulfide, along with wurtzite, generally 

 contains from less than 0.1 percent to 1 percent Cd 

 and locally as much as several percent. The world 



average for zinc concentrates, which are dominantly 

 sphalerite, is estimated as 0.24 percent Cd in material 

 containing 55 percent Zn (table 20) . The range and 

 average cadmium content of sphalerites from se- 

 lected U.S. and Canadian zinc mines and a few other 

 occurrences of significance are listed in table 21. 

 Higher temperature sphalerites appear to contain 

 less cadmium than those formed at lower tempera- 

 tures. Some of the lower cadmium values may also 

 be due to metamorphism — that is, cadmium is 

 mobilized and more widely dispersed preferentially 

 to zinc during metamorphism of a preexisting sulfide 

 deposit. 



RESOURCES 



Cadmium resources are closely associated with 

 zinc resources. If the Zn:Cd ratio of 455:1 based on 

 crustal abundances of these elements (table 20) is 

 valid, then the amount of cadmium in the uppermost 

 1 kilometer of the continental crust of the earth is 

 about 750 X 10' mt of which about 44 X 10" mt is 

 within this depth in the United States. Erickson (See 

 chapter on "Crustal Abundance") estimates world 

 and U.S. zinc reserve potentials as 3,400 X 10" and 

 198 X 10" mt, respectively. If Erickson's zinc poten- 

 tials are presumed to be chiefly in conventional sul- 

 fide ores, then similar cadmium reserve potentials, 

 based on a Zn : Cd ratio of 230 for world zinc concen- 

 trates (table 20) can be estimated as approximately 

 15 X 10" and 0.86 X 10" mt for the world and the 

 United States, respectively. 



Estimates of cadmium resources for the United 

 States and the world for this report are listed in 

 table 22. These estimates were prepared chiefly by 



