Arsenic crustal abundance

Uranium is not a very rare element. It is widely disseminated in nature with estimates of its average abundance in the Earth s crust varying from 2 to 4 ppm, close to that of molybdenum, tungsten, arsenic, and beryllium, but richer than such metals as bismuth, cadmium, mercury, and silver its crustal abundance is 2.7 ppm. The economically usable tenor of uranium ore deposits is about 0.2%, and hence the concentration factor needed to form economic ore deposits is about 750. In contrast, the enrichment factors needed to form usable ore deposits of common metals such as lead and chromium are as high as 3125 and 1750, respectively. 

The average crustal abundance of arsenic is 1.5mgkg . The element is strongly chaloco-phile. Approximately 60% of natural arsenic minerals are arsenates, 20% sulfides and sulfosalts, and the remaining 20% are arsenides. 

The average crustal abundance of selenium is 0.05mgkg (Jacobs, 1989). Like arsenic, selenium is strongly chalcophile and is partitioned into sulfides and rare selenides, such as... 

With regard to geochemical cycling (as well as for economic considerations), it is important to distinguish between the abundance of an element and its availability. The availability of an element is related not only to its relative abundance on Earth but also the stability of minerals in which it is a major constituent. Thus, a number of elements (e.g. copper, mercury, tin, and arsenic) that are scarce in terms of their average crustal abundance are easily isolated due to their ability to form mineral deposits. The most unavailable elements are those that form no major minerals of their own. Many of the rarer elements are available for economic use only to the extent that they are obtained as byproducts of the extraction of more abundant elements. Tellurium, for example, is produced during the electrolytic refining of copper. 

Elevated arsenic concentrations in soils or aquifer minerals are not, however, required to support dissolved arsenic concentrations in the range of a few to hundreds of fg/L. For a solid with an arsenic content (M) of 1.8 mg/kg (i.e., equal to the crustal abundance), the fraction (f) of arsenic that would need to be released to support a given dissolved arsenic concentration in the contacting pore water ([AsJdiss in Xg/L) can be calculated from the equation ... 

Nitrogen is the most abundant pnictide element in the solar system, in the sea, in the air, in the soil and in living organisms. It does, however, appear to take second place to phosphorus in the earth s crustal rocks (Table 2.4). The remaining pnictide elements, arsenic, antimony and bismuth, are all present in considerably smaller quantities than either nitrogen or phosphorus in all these media (Tables 2.2 and 2.5). 

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