U
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lytical chemistry The low concentrations of U in common rocks and minerals has meant that few techniques are available for use. Neutron activation and isotope dilution followed by thermal ionization mass spectrometry were the only choices until very recently, when inductively coupled plasma mass spectrometry became increasingly popular due to its extremely low detection limits (about 0.0005 J.tg/1) and ease of use. Colorimetric techniques coupled with flow-injection analysis are also being found of interest, due to low cost, good analytical performance, and reasonably low detection limits (7 J.lg/1; Perez-Pav6n et al., 1990).
Chemical properties Uranium is electropositive and reactive. It stains rapidly in air, becoming coated by a layer of oxide. The metal is attacked by acids but unaffected by alkalis. In HCl it leaves a residuum composed of UH(OH)z. Concentrated HN0 3 passivates U, but a mixture of HN0 3 and HF easily dissolves it. Cold water
attacks finely divided metallic U. The most stable oxidation state of uranium in aqueous solutions is 6 +. U 6 + forms the highly soluble uranyl ion uo~+, which behaves like divalent metal ions of smaller size (or the same size but higher charge) and is complexed by p-, OH-, So~-, N03, and carboxylates. The other oxidation state for which U is stable in aqueous solutions is 4 +. U 4 + forms a wide range of strong complexes with a high coordination number and varied geometry, those with fluorides and 0-donor chelating ligands being particularly numerous (Greenwood and Earnshaw, 1984).
Crystallochemical properties Uranium chiefly appears in minerals with valences 4 + and 6 +. Valence 5 + is quite unusual, and valence 3 + has never been found. The coordination number of oxygen around U 4 + is six or eight (U 4 + radii 0.89 A and 1.00 A, respectively). The coordination number of U 6 + around oxygen is six, seven, or eight (U 6 + radii 0.73 A, 0.81 A, and 0.83 A, respectively). Crystallochemical properties of U 4 + are very close to those of Th4 + (ionic radii 0.94 A and 1.05 A, for coordination six and eight respectively) and LREE3+ (ionic radii 1.03-0.98 A and 1.16-1.10 A, from La to Nd for coordination six and eight respectively). Since oxygen fugacity in most igneous processes is low enough to maintain all uranium as U 4 +, the geochemistry of this element in igneous rocks is strongly coherent with that of Th and LREE. In hydrothermal and supergene processes, however, uranium is partially or totally oxidized to U 6 +, and does not bear any coherence with the above elements.
Nuclear properties There are three natural uranium isotopes: 238 U (abundance 99.276%; half-life 4.468 x 10 9 years), 235 U (abundance 0.7196%; half-life 7.038 x 108 years), and 234 U (abundance 0.0057%; half-life 2.45 x 10 5 years). 238 U and 235 U are the parent isotopes of two different decay series having 236 Pb and 237 Pb as end products respectively. Due to its short half-life, no 234 U has survived from the Earth's formation, this isotope being continuously formed as an equilibrium intermediate p
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