Structural, mechanical, spectroscopic and thermodynamic characterization of the copper-uranyl tetrahydroxide mineral vandenbrandeite.

Abstract

The full crystal structure of the copper-uranyl tetrahydroxide mineral (vandenbrandeite), including the positions of the hydrogen atoms, is established by the first time from X-ray diffraction data taken from a natural crystal sample from the Musonoi Mine, Katanga Province, Democratic Republic of Congo. The structure is verified using first-principles solid-state methods. From the optimized structure, the mechanical and dynamical stability of vandenbrandeite is studied and a rich set of mechanical properties are determined. The Raman spectrum is recorded from the natural sample and determined theoretically. Since both spectra have a high-degree of consistence, all spectral bands are rigorously assigned using a theoretical normal-coordinate analysis. Two bands in the Raman spectra, located at 2327 and 1604 cm−1, are recognized as overtones and a band at 1554 cm−1 is identified as a combination band. The fundamental thermodynamic functions of vandenbrandeite are computed as a function of temperature using phonon calculations. These properties, unknown so far, are key-parameters for the performance-assessment of geological repositories for storage of radioactive nuclear waste and for understanding the paragenetic sequence of minerals arising from the corrosion of uranium deposits. The thermodynamic functions are used here to determine the thermodynamic properties of formation of vandenbrandeite in terms of the elements and the Gibbs free-energies and reaction constants for a series of reactions involving vandenbrandeite and a representative subset of the most important secondary phases of spent nuclear fuel. Finally, from the thermodynamic data of these reactions, the relative stability of vandenbrandeite with respect to these phases as a function of temperature and in the presence of hydrogen peroxide is evaluated. Vandenbrandeite is shown to be highly stable under the simultaneous presence of water and hydrogen peroxide.