Thermodynamics of formation of coffinite, USiO ₄

Abstract

Coffinite, USiO 4 , is an important U(IV) mineral, but its thermodynamic properties are not well-constrained. In this work, two different coffinite samples were synthesized under hydrothermal conditions and purified from a mixture of products. The enthalpy of formation was obtained by high-temperature oxide melt solution calorimetry. Coffinite is energetically metastable with respect to a mixture of UO 2 (uraninite) and SiO 2 (quartz) by 25.6 ± 3.9 kJ/mol. Its standard enthalpy of formation from the elements at 25°C is −1,970.0 ± 4.2 kJ/mol. Decomposition of the two samples was characterized by X-ray diffraction and by thermogravimetry and differential scanning calorimetry coupled with mass spectrometric analysis of evolved gases. Coffinite slowly decomposes to U 3 O 8 and SiO 2 starting around 450°C in air and thus has poor thermal stability in the ambient environment. The energetic metastability explains why coffinite cannot be synthesized directly from uraninite and quartz but can be made by low-temperature precipitation in aqueous and hydrothermal environments. These thermochemical constraints are in accord with observations of the occurrence of coffinite in nature and are relevant to spent nuclear fuel corrosion.uranium | coffinite | USiO 4 | U(IV) minerals | calorimetry I n many countries with nuclear energy programs, spent nuclear fuel (SNF) and/or vitrified high-level radioactive waste will be disposed in an underground geological repository. Demonstrating the long-term (10 6 -10 9 y) safety of such a repository system is a major challenge. The potential release of radionuclides into the environment strongly depends on the availability of water and the subsequent corrosion of the waste form as well as the formation of secondary phases, which control the radionuclide solubility. Coffinite (1), USiO 4 , is expected to be an important alteration product of SNF in contact with silica-enriched groundwater under reducing conditions (2-8). It is also found, accompanied by thorium orthosilicate and uranothorite, in igneous and metamorphic rocks and ore minerals from uranium and thorium sedimentary deposits (2,4,5,(8)(9)(10)(11)(12)(13)(14)(15)(16). Under reducing conditions in the repository system, the uranium solubility (very low) in aqueous solutions is typically derived from the solubility product of UO 2 . Stable U(IV) minerals, which could form as secondary phases, would impart lower uranium solubility to such systems. Thus, knowledge of coffinite thermodynamics is needed to constrain the solubility of U(IV) in natural environments and would be useful in repository assessment.In natural uranium deposits such as Oklo (Gabon) (4,7,11,12,14,17,18) and Cigar Lake (Canada) (5, 13, 15), coffinite has been suggested to coexist with uraninite, based on electron probe microanalysis (EPMA) (4,5,7,11,13,17,19,20) and transmission electron microscopy (TEM) (8, 15). However, it is not clear whether such apparent replacement of uraninite by a coffinite-like phase is a direct solid-state process or occurs mediated by dis...

Thermodynamic studies of studtite thermal decomposition pathways via amorphous intermediates UO3, U2O7, and UO4

“…Acquired data were processed with the Calisto software package from AKTS. Detailed procedures have been described previously [11,22].…”

“…Flushing oxygen gas at ~ 50 mL/min through the calorimeter chamber assisted in maintaining a constant gas environment above the solvent [25]. Dissolution of uranium oxides and other uranium-containing compounds as U 6+ species has been demonstrated in this solvent, and their drop solution enthalpy data were obtained previously [10,11,22,[26][27][28]. Upon rapid and complete dissolution of the sample, the enthalpy of drop solution, ΔH ds , was obtained.…”

Force percolation of contractile active gels

“…Motor activity can enhance network elasticity 61,68,138,227,238 , which may stabilize networks against motor-induced fracture. Furthermore, at high loads, muscle and non-muscle myosin II motors exhibit catch bond behavior [239][240][241] . Boundary adhesion may also need to be overcome.…”

“…Moreover, many myosins (skeletal muscle, smooth muscle, and non-muscle myosin IIB) exhibit catch bond behavior. Singlemolecule measurements with optical tweezers showed that the binding affinity of myosin motors to actin increases with applied force, for forces up to ~6 pN 239,240,335 . This catchbond behavior may promote contractility 336 .…”

Geochronology and Geochemistry of Uraninite and Coffinite: Insights into Ore-Forming Process in the Pegmatite-Hosted Uraniferous Province, North Qinling, Central China

“…Uraninite is capable of retaining the imprints of the earlier magmatic event and capturing the footprints of subsequent hydrothermal events [18,19]. On the other hand, recent researches suggest that mobilized U can be subsequently reprecipitated as coffinite during later silica-rich fluid circulation events [10,20]. Accordingly, the evolution of U mineral generations is instructive for understanding U circulation from magmatic to hydrothermal stages during pegmatite development.…”