Etude expérimentale du comportement de l'uranium dans les magmas: États d'oxydation et coordinance

Depending upon oxygen fugacity, uranium exists in three different oxidation states in magmatic silicate liquids. The hexavalent state, present as the uranyl group, UO²⁺₂, is stable under highly oxidizing conditions, but can still be detected in the presence of the NiNiO buffer. Under the same conditions the pentavalent state forms about 30–40% of total uranium and is also characteristic of relatively high oxygen fugacities. Optical absorption spectra obtained on granitic and basaltic glasses synthesized in the presence of the NiNiO buffer are very different: this is interpreted as being due to the presence of UO⁺₂ complexes in the former and 6-coordinated U(V) in the latter. The tetravalent state is the most stable under reducing conditions: at the FeFeO buffer, it is the only one present. An 8-coordinated U(IV) species seems the most probable, by comparison of the spectra with those of crystallized U(IV) compounds. The trivalent state was not detected, even under the most reducing conditions. Interpretation of the spectra obtained in the glasses in terms of coordination and bonding is however difficult, due to the lack of knowledge of 5f-systems in iono-covalent systems such as oxide glasses. The presence of the pentavalent state must be taken into account in discussing partition coefficients of uranium and trans-uranium compounds in natural and synthetic systems (because of the effect of oxygen fugacity and oxide ion activity on the U(IV) U(V) system). During postmagmatic hydrothermal processes U(V) is destroyed, resulting in the early precipitation of U(IV) containing minerals and possible migration of uranyl ions.