634 



UNITED STATES MINERAL RESOURCES 



The crustal abundance of thallium is low, approxi- 

 mating 1 ppm (part per million). Estimates of the 

 average crustal abundance of thallium range from 

 3 ppm (Ahrens, 1947b) to values as low as 0.1 ppm 

 (Noddack and Noddack, 1934). An average value 

 for all estimates approximates 1 ppm, which is in 

 agreement with the values of 1.3 ppm given by- 

 Shaw (1952, 1957) and of 1-1.1 ppm by Ivanov (in 

 Vlasov, 1964). Although the abundance of thallium 

 is extremely low, it is in excess of the crustal abun- 

 dances of elements such as mercury, silver, and gold. 

 Unlike these elements, however, thallium rarely 

 concentrates into specific minerals. Several char- 

 acteristic thallium sulfide, selenide, and oxide min- 

 erals occur in nature, but they are extremely rare; 

 most thallium occurs as a trace element in other 

 minerals. 



The occurrence and concentration of thallium ap- 

 pear to be functions of genetic environment and 

 elemental association. These conditions, in turn, are 

 governed by chemical affinity, atomic size, valency, 

 and temperature and pressure. 



DISTRIBUTION IN MINERALS AND ROCKS 



The main source of thallium metal is sulfide ore, 

 mined primarily for base metals such as lead and 

 zinc that yield thallium as a byproduct. The thallium 

 content of these sulfides ranges from 10,000 ppm to 

 nondetectable amounts. The most consistent occur- 

 rence of thallium is in the structure of the alkali 

 feldspars and some micas, in which values range 

 from 600 ppm to nondetectable amounts; these sili- 

 cate minerals have little or no present economic 

 significance. 



Vlasov (1964) noted that although thallium occurs 

 in sphalerite, pyrite, marcasite, chalcopyrite, pyr- 

 rhotite, and other sulfide minerals, it is particularly 

 concentrated in the colloform varieties. Thus it may 

 be possible to explain the wide variation of thallium 

 content in specific sulfide minerals in terms of tem- 

 perature and pressure — those sulfides formed from 

 a crystallizing melt generally being low in thallium, 

 and those formed from colloidal suspensions under 

 lower temperatures and pressures generally being 

 enriched in thallium, with intermediate variations 

 between these end members. 



Because of chemical affinity, size, and valency, 

 thallium associates with silicate minerals rich in the 

 alkali metals potassium, rubidium, and cesium. The 

 isomoi-phous substitution of thallium for potassium 

 is readily attained in the alkali feldspars and in the 

 micas where potassium is contained as the relatively 

 loosely bound, high coordinated interlayer cation. 

 Hence their thallium content may be quite high, in 



the hundreds of parts per million, especially in the 

 later-stage pegmatitic alkali feldspars and micas. 



In minerals of sedimentary origin, many manga- 

 nese hydroxides contain concentrations of thallium 

 with values as high as 2,300 ppm. Substantial thal- 

 lium content has also been recorded in pyrites of 

 sedimentary origin, particularly those associated 

 with coal beds. 



Thallium is widely dispersed in rocks, but its dis- 

 tribution is controlled by its general restriction to 

 the sulfide and alkali-rich silicate rock-forming min- 

 erals. Thus its concentration in ultrabasic rocks is 

 very low, less than 1 ppm, increasing gradually 

 through intermediate to acidic and pegmatitic rocks 

 to concentrations greater than 1 ppm. Thallium ap- 

 pears to increase in direct relationship to an increase 

 in alkali feldspars and potassium-rich micas. In 

 particular, it is noted to concentrate in the later 

 stage, lower temperature pegmatites and aplites. 



The relationship of thallium to metamorphic rocks 

 is poorly understood. On the basis of characteristic 

 mineral assemblages, particularly those rich in the 

 alkali metals, it would appear logical for thallium to 

 decrease with increasing metamorphic grade in an 

 open system. 



In the sedimentary cycle, the behavior of thallium 

 is complex. Thallium may occur in a detrital grain 

 of the original host mineral, such as a feldspar or 

 mica. It may also occur as a constituent of those 

 minerals formed in the sedimentary environment — 

 in highly oxidizing conditions incorporated into 

 manganese hydroxides, in reducing conditions as a 

 sulfide in colloidal pyrite, and in a marine environ- 

 ment preferentially absorbed by colloidal clays and 

 shales. 



Thus thallium values range from 6 ppm, recorded 

 in a shale, to nondetectable values in carbonate 

 rocks. 



RESOURCES 



World reserves and potential resources of thallium 

 (table 131) were calculated from respective com- 

 modity reserve and resource estimates prepared for 

 this volume (except for iron sulfides, which were 

 based on U.S. Bureau of Mines estimates), using 

 thallium contents of 2.2 ppm in zinc sulfides, 2 ppm 

 in lead sulfides, 0.25 ppm in iron sulfides, 0.75 ppm 

 in coal ash, and 10 ppm in manganese nodules — 

 factors computed from recently processed analytical 

 data of a study of the distribution and occurrence 

 of thallium in rocks and minerals. 



The United States is potentially self-sufficient in 

 thallium resources, the reserves contained in zinc, 

 lead, and iron sulfides being 266 short tons, and 



