THALLIUM 



633 



material. As a group, potassium-rich silicates con- 

 tain the greatest and most consistent amounts of 

 thallium, but these minerals have little or no com- 

 mercial value in the metallurgical processing indus- 

 try at this time. Thus, as Greenspoon (1970) stated, 

 the exploitation of thallium is an integral function 

 of zinc production and demand. To date, world con- 

 sumption of thallium has been less than the amount 

 that could be produced as a byproduct from base- 

 metal sulfides. 



Except for two periods of marked increase in 

 demand, one in the late-middle 1950's and one in 

 the early-middle 1960's, U.S. consumption of thal- 

 lium has been relatively stable, approximating 4,000 

 pounds annually. Greenspoon (1970), however, indi- 

 cated that the 1968 demand for thallium had risen 

 to 6,500 pounds. She gave values of annual peak 

 demand for thallium in the mid-1950's at approxi- 

 mately 14,000 pounds and in the mid-1960's at some 

 9,000 pounds. The 1950's demand probably resulted 

 from increased research using radiation equipment 

 and from sophistication of military defense systems, 

 particularly infrared detection. The increased de- 

 mand in the mid-1960's may have been due to the 

 conversion of domestic electronic equipment to solid 

 state and increased use of thallium in or associated 

 with electronic components. The sudden drop follow- 

 ing this increase in demand was probably due to the 

 Federal curtailment on the use of thallium as a 

 rodenticide. From 1964 to 1968 no thallium was pro- 

 duced in the United States, presumably because 

 processed reserves were sufficient to meet require- 

 ments. Thus, production is sporadic. It is likely that 

 industrial plants that use thallium buy sufficient 

 amounts at any one time to last several years. As a 

 consequence, production/time curves are erratic and 

 of no meaningful significance. 



GEOLOGIC ENVIRONMENT 



GEOCHEMISTRY 



Thallium, atomic number 81, atomic weight 204.39, 

 occurs next to lead in the periodic table and the two 

 metals exhibit similar physical properties. There are 

 two naturally occurring stable isotopes of thallium, 

 TP"^ and Tl-°^ their relative abundances being given 

 as 29.46 and 70.54 percent respectively. Thallium 

 metal is bluish white to silvery white and, like lead, 

 is soft and malleable. Unlike lead, however, a fresh- 

 cut surface of thallium metal will develop a thick 

 oxide coating in several weeks. The density of 

 thallium is 11.85 g/cm' at 20°C, the melting point 

 is 303°C, and the boiling point is 1,457°C. Thallium 

 metal occurs with three crystal lattice modifications : 



below 230° C it is hexagonal close-packed, above 

 230°C it is body-centered cubic, and at high pres- 

 sures it is face-centered cubic, the triple point being 

 at 110°C and 30 kilobars. 



Shannon and Prewitt (1969) gave the following 

 ionic radii for thallium in various coordination 

 polyhedra. 



Thallium was placed in group III of the periodic 

 table by Mendeleev, under boron, aluminum, gallium, 

 and indium. Thallium has three electrons in its 

 outer valence shell and the maximum positive va- 

 lence state attained is T1 + ', by donation of the two 

 outer 6s^ and single 6p^ electrons to coordinating 

 anions. The thallic state is generally attained under 

 highly oxidizing conditions. The single 6p' electron, 

 however, is much more readily donated than the 

 two 6s" electrons, so that under more normal endo- 

 gene processes, thallium is preferably in the mono- 

 valent T1 + ' or thallous state. Thallous (Tl+i) salts 

 are generally highly volatile. 



In the monovalent state, thallium has unique 

 chemical and physical properties, resembling both 

 lead and the alkali metals potassium, rubidium, and 

 cesium. Thus, thallium is both a chalcophile and a 

 lithophile element, and under certain conditions 

 displays considerable siderophilic affinities. As a 

 consequence it is widely dispersed in the earth's 

 crust. Vlasov (1964, p. 491-680 of English transla- 

 tion) stated that thallium metal closely resembles 

 lead in density, melting point, electrical conductivity, 

 hardness, atomic volume, radius, and thermal sta- 

 bility. Like lead and other chalcophile elements, 

 thallium is also diamagnetic. Similarly, monovalent 

 thallium resembles the alkali metals potassium, ru- 

 bidium, and cesium of the lithophile group in both 

 physical and chemical properties. It replaces K+' 

 by isomorphous substitution in silicates such as the 

 alkali feldspars and micas, and in some minerals 

 such as pollucite it is thought to replace Rb+' and 

 Cs+' (Ahrens, 1947a). Several authors have at- 

 tempted, with limited success, to establish Rb/Tl 

 and K/Tl ratios on basic through acidic and peg- 

 matitic igneous rocks. In some instances there is a 

 suggestion of a direct linear relationship between 

 rubidium and thallium (Ahrens, 1948) and the sug- 

 gestion of an inverse linear relationship between 

 potassium and thallium. 



