Guo Xiaofeng, Szenknect Stéphanie, Mesbah Adel, Labs Sabrina, Clavier Nicolas, Poinssot Christophe, Ushakov Sergey V, Curtius Hildegard, Bosbach Dirk, Ewing Rodney C, Burns Peter C, Dacheux Nicolas, Navrotsky Alexandra
Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616; Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545;
Institut de Chimie Séparative de Marcoule, UMR 5257, CNRS/CEA/Université Montpellier/Ecole Nationale Supérieure de Chimie de Montpellier, Site de Marcoule, 30207 Bagnols sur Cèze, France;
Proc Natl Acad Sci U S A. 2015 May 26;112(21):6551-5. doi: 10.1073/pnas.1507441112. Epub 2015 May 11.
Coffinite, USiO4, 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 UO2 (uraninite) and SiO2 (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 U3O8 and SiO2 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.
硅铀矿（USiO₄）是一种重要的四价铀矿物，但其热力学性质尚未得到很好的确定。在这项工作中，在水热条件下合成了两种不同的硅铀矿样品，并从产物混合物中进行了提纯。通过高温氧化物熔体溶液量热法获得了生成焓。相对于UO₂（晶质铀矿）和SiO₂（石英）的混合物，硅铀矿在能量上是亚稳的，其差值为25.6±3.9 kJ/mol。其在25℃时由元素形成的标准焓为-1970.0±4.2 kJ/mol。通过X射线衍射、热重分析以及与逸出气体的质谱分析相结合的差示扫描量热法对这两种样品的分解进行了表征。硅铀矿在空气中从约450℃开始缓慢分解为U₃O₈和SiO₂，因此在环境中热稳定性较差。能量亚稳性解释了为什么硅铀矿不能直接由晶质铀矿和石英合成，而可以通过在水溶液和水热环境中的低温沉淀来制备。这些热化学限制与自然界中硅铀矿的出现情况相符，并且与乏核燃料腐蚀相关。
