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Title: Thermodynamics of formation of coffinite, USiO₄

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:
 [1] ;  [2] ;  [2] ;  [3] ;  [2] ;  [4] ;  [5] ;  [3] ;  [3] ;  [6] ;  [7] ;  [2] ;  [5]
  1. Univ. of California, Davis, CA (United States); Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
  2. Univ. Montpellier, Bagnols sur Ceze cedex (France)
  3. Nuclear Waste Management, Julich (Germany)
  4. DRCP/DIR, Bagnols sur Ceze cedex (France)
  5. Univ. of California, Davis, CA (United States)
  6. Stanford Univ., Stanford, CA (United States)
  7. Univ. of Notre Dame, Notre Dame, IN (United States)
Publication Date:
Report Number(s):
LA-UR-15-22412
Journal ID: ISSN 0027-8424
Grant/Contract Number:
AC52-06NA25396; DESC0001089
Type:
Published Article
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Volume: 112; Journal Issue: 21; Journal ID: ISSN 0027-8424
Publisher:
National Academy of Sciences, Washington, DC (United States)
Research Org:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; 36 MATERIALS SCIENCE; Material Science; Uranium; Coffinite; USiO4; U(IV) minerals; Calorimetry
OSTI Identifier:
1235181
Alternate Identifier(s):
OSTI ID: 1200681