Synthesis of Coffinite--USiO4

Hoekstra, H.R.; Fuchs, Louis H.

doi:10.1126/science.123.3186.105

Cited by 33 publications

(38 citation statements)

References 1 publication

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This is consistent with previous reports (24,30). After heating to 1,000°C in air, no trace of USiO 4 is detected in either sample.…”

Section: Resultssupporting

confidence: 94%

“…As a result, the formation of a uranothorite phase with x > 0.26 (coffinite included) is suggested to be thermodynamically unfavorable (40). In addition, a Th-U dioxide phase and amorphous silica were inevitably present in these products (24,30).…”

Section: Resultsmentioning

confidence: 99%

“…It appears not to form by reacting the oxides under dry high-temperature conditions (24, 29). Synthesis from aqueous solutions usually produces UO 2 and amorphous SiO 2 impurities, with coffinite sometimes being only a minor phase (24,(30)(31)(32)(33)(34)(35). It is not clear whether these difficulties arise from kinetic factors (slow reaction rates) or reflect intrinsic thermodynamic instability (33).…”

mentioning

confidence: 99%

See 2 more Smart Citations

Thermodynamics of formation of coffinite, USiO ₄

Guo

Szenknect

Mesbah

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

107

View full text Add to dashboard Cite

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...

show abstract

“…This is consistent with previous reports (24,30). After heating to 1,000°C in air, no trace of USiO 4 is detected in either sample.…”

Section: Resultssupporting

confidence: 94%

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

See 1 more Smart Citation

Thermodynamics of formation of coffinite, USiO ₄

Guo

Szenknect

Mesbah

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

107

View full text Add to dashboard Cite

show abstract

“…Coffinite was first described as an independent mineral phase in 1956 [45,53] and is reported with a composition of USiO 4 AE nH 2 O. This matches well with the constant ratio U-Si of 1:1 observed with one of the phases.…”

Section: Discussion On Potential U(iv) Secondary Phasessupporting

confidence: 74%

“…Here, equally, no defined crystals were identified. Since coffinite is known to form at hydrothermal conditions in nature [44], and since it was possible to synthesize it under laboratory conditions [45,46], its formation during our experiments seems plausible. The observation that the macroscopic structure of the leached UO 2 material is changed (visible as cracks where a high abundance of Si was detected on the surface) supports the assumption of phase transformation.…”

Section: Concentration Profiles and Surface Alterations Under Hydrothmentioning

confidence: 98%