title: Thermodynamics of formation of coffinite, USiO4.
abstract: 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.
authors.affiliation: 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;
authors.forename: Xiaofeng
authors.initials: X
authors.lastname: Guo
authors.affiliation: 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;
authors.forename: Stéphanie
authors.initials: S
authors.lastname: Szenknect
authors.affiliation: 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;
authors.forename: Adel
authors.initials: A
authors.lastname: Mesbah
authors.affiliation: Institute of Energy and Climate Research, Nuclear Waste Management, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany;
authors.forename: Sabrina
authors.initials: S
authors.lastname: Labs
authors.affiliation: 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;
authors.forename: Nicolas
authors.initials: N
authors.lastname: Clavier
authors.affiliation: CEA, Nuclear Energy Division, RadioChemistry & Processes Department, BP 17171, 30207 Bagnols sur Cèze, France;
authors.forename: Christophe
authors.initials: C
authors.lastname: Poinssot
authors.affiliation: Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616;
authors.forename: Sergey V
authors.initials: SV
authors.lastname: Ushakov
authors.affiliation: Institute of Energy and Climate Research, Nuclear Waste Management, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany;
authors.forename: Hildegard
authors.initials: H
authors.lastname: Curtius
authors.affiliation: Institute of Energy and Climate Research, Nuclear Waste Management, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany;
authors.forename: Dirk
authors.initials: D
authors.lastname: Bosbach
authors.affiliation: Department of Geological and Environmental Sciences, School of Earth Sciences, Stanford University, Stanford, CA 94305; and.
authors.forename: Rodney C
authors.initials: RC
authors.lastname: Ewing
authors.affiliation: Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN 46556.
authors.forename: Peter C
authors.initials: PC
authors.lastname: Burns
authors.affiliation: 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;
authors.forename: Nicolas
authors.initials: N
authors.lastname: Dacheux
authors.affiliation: Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616; anavrotsky@ucdavis.edu.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Navrotsky
keywords: U(IV) minerals; USiO4; calorimetry; coffinite; uranium