SREL Reprint #2494

Dehydration of synthetic autunite hydrates

By A. G. Sowder¹, S. B. Clark²  and R. A. Fjeld³

¹The University of Georgia, Savannah River Ecology Laboratory, Aiken, South Carolina 29802, USA
²Washington State University, Department of Chemistry, Pullman, Washington 99164, USA
³Clemson University, Department of Environmental Engineering and Science, Clemson, South Carolina 29634, USA

(Received September 20, 1999; accepted April 3, 2000)

Autunite / Metaautunite / Dehydration / Uranyl phovphate                   

Summary.    The dehydration of uranyl minerals can affect phase structure and stability. Synthetic autunite hydrates, Ca[(UO₂)(PO₄)]₂.xH₂O, were studied by X-ray powder diffractometry (XRD) and thermogravimetric analysis (TGA) to address ambiguous or contradictory reports in the literature.  Structurally, XRD analysis supported the three well-defined phases commonly reported in the literature, i.e. autunite, metaautunite I, and metaautunite II.   In addition, a fourth phase with a basal plane spacing between that of autunite and metaautunite I, designated metaautunite Ia, was identified as an apparent metastable intermediate. TGA analysis confirmed that water loss or accumulation is tolerated to different degrees among the autunite hydrates. Loss of low temperature water appears to initiate collapse of the interlayer spacing from 10 to 9 Å to form metaautunite I and/or Ia, while the lower hydrates accommodate minor water loss and accumulation without significant structural alteration. Our results support  previous research indicating the reversibility of the autunite to metaautunite I conversion. The complex dehydration pattern                of autunite is not observed in all the 1: 1 uranyl phosphates, such as chernikovite (H[(UO₂)(PO₄)].4H₂0).    

SREL Reprint #2494

Sowder, A. G., S. B. Clark, and R. A. Fjeld. 2000. Dehydration of synthetic autunite hydrates. Radiochim. Acta 88:533-538.

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