Selenium abundance

Where peridotites show both textural and geochemical evidence for pervasive interaction with percolating melts, PGE and selenium abundances are greatly reduced. PGE abundances decrease by —80%, probably because of dissolution of intergranular sulhdes (Lorand and Alard, 2001). 

Selenium is a rare element in the earth s crust, but is of considerable interest due to its ready incorporation in the food chain. Selenium is most frequently found in base metal ores of lead, copper, and nickel. Selenium abundance in the earth s crust is estimated to be 0.05 mg kg and selenium... 

We also developed a number of other useful new fluorinating reagents. They ineluded a convenient in situ form of sulfur tetrafluoride in pyridinium polyhydrogen fluoride, selenium tetrafluoride, and ey-anurie fluoride. We introdueed uranium hexafluoride (UFg), depleted from the U-235 isotope, which is an abundant by-product of enrichment plants, as an effective fluorinating agent. 

Comprehensive accounts of the analytical chemistry of teUurium have been pubUshed (5,26—30). The analytical methods for the determination of teUurium are to a considerable extent influenced by the element s resemblance, in many of its properties and in its limited terrestrial abundance, to selenium. 

The only sulfur isotope with a nuclear spin is which is quadrupolar (/ = 3/2) and of low natural abundance (0.76%). In view of these inherent difficulties and the low symmetry around the sulfur nuclei in most S-N compounds, S NMR spectroscopy has found very limited application in S-N chemistry. However, it is likely that reasonably narrow resonances could be obtained for sulfur in a tetrahedral environment, e.g. [S(N Bu)4], cf. [S04] . On the other hand both selenium and tellurium have isotopes with I = Vi with significant natural abundances ( Se, 7.6% and Te, 7.0%). Consequently, NMR studies using these nuclei can provide useful information for Se-N and Te-N systems. 

Abundances of lUPAC (the International Union of Pure and Applied Chemistry). Their most recent recommendations are tabulated on the inside front fly sheet. From this it is clear that there is still a wide variation in the reliability of the data. The most accurately quoted value is that for fluorine which is known to better than I part in 38 million the least accurate is for boron (1 part in 1500, i.e. 7 parts in [O ). Apart from boron all values are reliable to better than 5 parts in [O and the majority arc reliable to better than I part in 10. For some elements (such as boron) the rather large uncertainty arises not because of experimental error, since the use of mass-spcctrometric measurements has yielded results of very high precision, but because the natural variation in the relative abundance of the 2 isotopes °B and "B results in a range of values of at least 0.003 about the quoted value of 10.811. By contrast, there is no known variation in isotopic abundances for elements such as selenium and osmium, but calibrated mass-spcctrometric data are not available, and the existence of 6 and 7 stable isotopes respectively for these elements makes high precision difficult to obtain they are thus prime candidates for improvement. 

Selenium, Te and Po are the three heaviest members of Group 16 and, like their congenors O and S, have two p electrons less than the next following noble gases. Selenium is normally said to have 6 stable isotopes though the heaviest of these ( Se, 8.73% abundant) is actually an extremely long-lived fi emitter. 

Zinc (76ppm of the earth s crust) is about as abundant as rubidium (78 ppm) and slightly more abundant than copper (68 ppm). Cadmium (0.16 ppm) is similar to antimony (0.2 ppm) it is twice as abundant as mercury (0.08 ppm), which is itself as abundant as silver (0.08 ppm) and close to selenium (0.05 ppm). These elements are chalcophiles (p. 648) and so, in the reducing atmosphere prevailing when the earth s crust solidified, they separated out in the sulfide phase, and their most important ores are therefore sulfides. Subsequently, as rocks were weathered, zinc was leached out to be precipitated as carbonate, silicate or phosphate. 

The most important minerals of sulfur The most abundant minerals of selenium and tellurium... 

The natural abundance of selenium-77 is 7.58%. The chemical shift of dimethyl selenide is set equal to 0 ppm. The total chemical shift range is around 2200 ppm, organoselenium compounds covering almost the whole range. Figure 41 shows the spectrum of H2Se03 in D20, the linewidth being only... 

Thallium is the 59th most abundant element found in the Earths crust. It is widely distributed over the Earth, but in very low concentrations. It is found in the mineral/ores of crooksite (a copper ore CuThSe), lorandite (TLAsS ), and hutchinsonite (lead ore, PbTl). It is found mainly in the ores of copper, iron, sulfides, and selenium, but not in its elemental metallic state. Significant amounts of thallium are recovered from the flue dust of industrial smokestacks where zinc and lead ores are smelted. 

ISOTOPES There are a total of 35 isotopes of selenium. Five of these are stable, and a sixth isotope has such a long half-life that it is also considered stable Se-82 = 0.83x10+20 years. This sixth isotope constitutes 8.73% of selenium s abundance in the Earth s crust, and the other five stable isotopes make up the rest of selenium s abundance on Earth. 

Selenium is the 67th most abundant element in Earths crust. It is widely spread over the Earth, but does not exist in large quantities. As a free element it is often found with the element sulfur. 

Because selenium to some extent is chemically similar to sulfur, one might expect to find some analogous fractionations of selenium isotopes in nature. Six stable selenium isotopes are known with the following abundances (Coplen et al. 2002)... 

Most of these are relatively common and some are common indeed. For example, silicon and aluminum are the second and third most abundant elements in the Earth s crust. The rarest of these elements, selenium, is twice as abundant as silver and 20 times more abundant than gold, and it is relatively easy to obtain because it often occurs in sulfur deposits. 

Selenium is a very rare element. The metal does not occur in nature in free elemental form. Its abundance in the earth s crust is about 0.05 mg/kg. It... 

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