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...

Metastudtite, (UO 2 )O 2 (H 2 O) 2 , is one of two known natural peroxide minerals, but little is established about its thermodynamic stability. In this work, its standard enthalpy of formation, −1,779.6 ± 1.9 kJ/mol, was obtained by high temperature oxide melt drop solution calorimetry. Decomposition of synthetic metastudtite was characterized by thermogravimetry and differential scanning calorimetry (DSC) with ex situ X-ray diffraction analysis. Four decomposition steps were observed in oxygen atmosphere: water loss around 220°C associated with an endothermic heat effect accompanied by amorphization; another water loss from 400°C to 530°C; oxygen loss from amorphous UO 3 to crystallize orthorhombic α-UO 2.9 ; and reduction to crystalline U 3 O 8 . This detailed characterization allowed calculation of formation enthalpy from heat effects on decomposition measured by DSC and by transposed temperature drop calorimetry, and both these values agree with that from drop solution calorimetry. The data explain the irreversible transformation from studtite to metastudtite, the conditions under which metastudtite may form, and its significant role in the oxidation, corrosion, and dissolution of nuclear fuel in contact with water. Metastudtite is the main natural peroxide mineral (1) due to the irreversible dehydration of studtite (2). Both these phases may be found in spent nuclear fuel exposed to water (3-5). Burns et al. (6), by single crystal diffraction of a natural sample, determined the crystal structure of studtite to be monoclinic with space group C2/c. The structure consists of edge-sharing UO 8 -polyhedra chains. The water molecules are located on two different positions between these chains. Although there are different, yet very similar, models for metastudtite, thus far no structure has been determined (7,8). It is highly probable that metastudtite, like studtite, consists of chains of edge-sharing UO 8 polyhedra with the water molecules located between the chains. To better depict the bonding situation and the different oxygen atoms, it is proposed to write the sum formulas as (UO 2 )(O 2 )(H 2 O) 2 ·2H 2 O for studtite and (UO 2 )(O 2 )(H 2 O) 2 for metastudtite [in analogy to Burns et al. (6)].Studtite and other polyoxouranylates have a potentially important role as alteration phases in geological repositories for nuclear waste, specifically regarding the interactions of nuclear waste with the aqueous environment (9, 10). They have also been proposed as corrosion products in sea water after the FukushimaDaiichi nuclear plant accident (10-12).In a nuclear waste repository, the high alpha dosage will be the dominant factor after the first thousand years of storage (13). In combination with groundwater, the alpha radiation will lead to formation of H 2 O 2 (3, 4, 9, 14). These very localized oxidative conditions could trigger the genesis of studtite or metastudtite even if the overall conditions are reducing (14). The formation of metastudtite on UO 2 samples as a direct effect of alpha radiolysis of water w...

The behavior of radionuclides in the environment (geo-, hydro-, and biosphere) is determined by interface reactions like adsorption, ion exchange, and incorporation processes. Presently, operational gross parameters for the distribution between solution and minerals are available. For predictive modeling of the radionuclide mobility in such systems, however, individual reactions and processes need to be localized, characterized, and quantified. A prerequisite for localization and clarification of the concerned processes is the use of modern advanced analytical and speciation methods, especially spectroscopy. In this study, Eu(III) was chosen as an analogue for trivalent actinides to identify the different species that occur by the Ln(III)/hydrotalcite interaction. Therefore, Eu(III) doped Mg-Al-Cl-hydrotalcite was synthesized and investigated by TRLFS, EXAFS, and XRD measurements. Two different Eu/hydrotalcite species were obtained. The minor part of the lanthanide is found to be inner-sphere sorbed onto the mineral surface, while the dominating Eu/hydrotalcite species consists of Eu(III) that is incorporated into the hydrotalcite lattice. Both Eu/hydrotalcite species have been characterized by their fluorescence emission spectra and lifetimes. Structural parameters of the incorporated Eu(III) species determined by EXAFS indicate a coordination number of 6.6 +/- 1.3 and distances of 2.41 +/- 0.02 A for the first Eu-OH shell.

The miscibility behavior of the USiO4-ThSiO4 system was investigated. The end members and 10 solid solutions UxTh(1-x)SiO4 with x = 0.12-0.92 were successfully synthesized, without formation of other secondary uranium or thorium phases. Lattice parameters of the solid solutions evidently follow Vegard's Law. Investigation of the local structure with EXAFS reveals small differences between the U and Th environment attributed to different atomic radii of the metal atoms but no implications for a miscibility gap. The data provided confirm complete miscibility for the system USiO4-ThSiO4. The structure of the end members was studied in detail with XRD and discussed with special regard to the oxygen positions and the often neglected Si-O bond length. USiO4 could be obtained without UO2 impurities and the lattice parameters derived from Rietveld refinement as c = 6.2606(3) Å and a = 6.9841(3) Å. The Si-O distance in USiO4 appears to be 1.64 Å, which is more reasonable than earlier reported values.