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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2018: 2.631

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
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1945-3027
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Volume 102, Issue 10

Issues

Thermodynamic characterization of synthetic autunite

Ewa A. Dzik
  • Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, U.S.A
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/ Haylie L. Lobeck
  • Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, U.S.A
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/ Lei Zhang
  • Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, U.S.A
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/ Peter C. Burns
  • Corresponding author
  • Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, U.S.A
  • Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, U.S.A
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Published Online: 2017-10-02 | DOI: https://doi.org/10.2138/am-2017-6109

Abstract

Autunite, Ca[(UO2)(PO4)]2(H2O)11, is a common uranyl mineral found in oxidized portions of uranium deposits, as well as subsurface environments contaminated by uranium. Enthalpies of formation of autunite were obtained via high-temperature oxide melt calorimetry using a 3Na2O⋅4MoO3 solvent at 976 K. The synthetic analog of autunite was prepared using slow mixing by diffusion into an aqueous barrier solution at room temperature. Prior to calorimetric measurements, the material was characterized using powder X-ray diffraction (PXRD), inductively coupled plasma optical emission spectrometry (ICP-OES), thermogravimetric analysis (TGA), and Raman spectroscopy, to ensure purity. The calculated enthalpy of formation from binary oxides of autunite is −579.92 ± 21.68 kJ/mol; the enthalpy of formation from the elements is −8311.32 ± 21.79 kJ/mol. The measured drop solution enthalpy allowed calculation of the enthalpy of the reaction of dehydration of autunite to meta-autunite. The results demonstrate that autunite is a metastable phase and explain the observed rapid dehydration to meta-autunite, a lower hydrate, as well as the common occurrence of the latter mineral in nature.

Keywords: Autunite; calorimetry; uranium; enthalpy; thermodynamics; metaphase; Actinides in Geology; Energy and the Environment

Special collection papers can be found online at http://www.minsocam.org/MSA/AmMin/special-collections.html.

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About the article

Received: 2017-02-13

Accepted: 2017-06-08

Published Online: 2017-10-02

Published in Print: 2017-10-26


Citation Information: American Mineralogist, Volume 102, Issue 10, Pages 1977–1980, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2017-6109.

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© 2017 by Walter de Gruyter Berlin/Boston.

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