Chemical and Physical Properties of Uranium
and Geological Processes
Uranium is the heaviest and last naturally occurring element in the periodic table, with an atomic number of 92 and an atomic mass of 238. Because of its large ionic radius and high charge, uranium does not enter in the structure of major rock-forming minerals, and consequently is continuously enriched in melts either during magmatic processes such as partial melting or fractional crystallization. As a result, the most fractionated magmas—which are generally the richest in silica—are the most enriched in uranium; granites and rhyolites are much richer in uranium than mafic igneous rocks such as basalts or gabbros. In igneous rocks, uranium is associated with enriched thorium (Th), zirconium (Zr), titanium (Ti), niobium (Nb), tantalum (Ta), and rare earth elements (in minerals such as zircon, apatite, monazite, titanite, allanite, uraninite, etc.), particularly in peralkaline rocks but less so for metaluminous rocks and much less for peraluminous rocks.
Levels of uranium in common sedimentary rocks are closely related to the oxidation-reduction conditions. The highest concentrations (tens to hundreds of parts per million [ppm]a) are found in sediments that are rich in organic matter or phosphate. Lower uranium contents are generally recorded in coarse-grained sediments, and higher values in clay-rich sediments.
Uranium in nature occurs in two main oxidation states, U4+ and U6+. The U4+ state is stable in reducing conditions, weakly soluble in most geological conditions, and is the main valence occurring in uranium ore minerals (dominantly tetravalent uranium minerals). U6+ forms the uranyl UO22+ species, which is stable in oxidizing conditions and forms a large series of complexes (hydroxides, carbonates, sulfates, phosphates, etc.) which are very soluble in geological fluids. The uranyl species enters into the structure of hexavalent uranium minerals, which are also called secondary uranium minerals because they commonly result from the oxidation of tetravalent uranium minerals by interaction with oxygen-bearing surficial waters.
Uranium minerals are extremely diverse. Approximately 5 percent of all known minerals contain uranium as an essential structural constituent (Burns, 1999), although many of the hundreds of uranium-bearing minerals are rarely encountered mineral “curiosities.” Among the tetravalent uranium minerals, the two principal ones occurring in ore deposits are uraninite, with a UO2+x composition (called pitchblende when occurring with a colloform texture), and coffinite (USiO4).
Other common tetravalent minerals that generally contain several percent to several tens of percent of uranium are uranothorite (Th,U)SiO4, brannerite (U,Ca,Ce) (Ti,Fe)2O6, ningyoite (U,Ca,Ce)2(PO4)2·1.5H2O, Nb-Ta-Ti minerals such as uran-microlite (U,Ca,Ce)2(Nb,Ta)2O6(OH,F), uranpyrochlore (U,Ca,Ce)2(Ta,Nb)2O6(OH,F), euxenite (Y, Er, Ce, La, U)(Nb, Ti, Ta)2(O,OH)6 and can be also associated with organic matter in thucolite. Hexavalent uranium minerals are less abundant in ore deposits, but are the most diverse. They are highly colored and can be deposited either as primary ore minerals such as carnotite K2(UO2)2(VO4)2·3H2O, tyuyamunite Ca(UO2)2(VO4)2·3H2O, or more commonly as alteration products of tetravalent uranium minerals such as autunite Ca(UO2)2(PO4)2·10H2O or uranophane Ca(UO2)2SiO3(OH)2·5H2O.