The Unique Association of Thallium and Rubidium in Minerals

Spectrochemical analyses of various minerals has shown that "alkali-metal" thallium and rubidium are found only in potassium minerals and the cesium mineral, pollucite, and that in these minerals the Tl: Rb association is very close. Altogether, 167 specimens have been analyzed quantitatively, the selection comprising lepidolite, amazonite, hydrothermal pegmatitic microcline, primary pegmatitic microcline, zinnwaldite, biotite, muscovite, phlogopite, pollucite, rhodizite, and cesium beryl. The mean weight percentage ratio of Rb₂O/Tl₂O was determined as 100; and the vast majority of the ratios fall within the limits of 35-300: the extreme limits are 10 and 650. A plot of log percentage of Rb₂O versus log percentage of Tl₂O produced a curve of unit slope over a thousand fold range of concentration that could be investigated. There thus appears to be no shift in the ratio Rb/Tl throughout the selective crystallization of minerals, and the ratio seems to be independent of type of host mineral; the ratio does, however, vary to some extent from area to area. The reasons for the close association of alkali-metal thallium and rubidium are that the radii of their ions are identical and that in certain pertinent respects their chemical properties are very similar. With the possible exception of the pair Zr:Hf, which are very closely associated in minerals, alkali-metal thallium and rubidium are perhaps the most closely associated pair of elements in the earth's crust, and their association is all the more remarkable because thallium is a Group ₃b element, whereas rubidium is an alkali metal (Group 1a). The abundance of thallium in the earth's crust has been estimated as 0.0003 per cent by weight.