Dehydration of synthetic autunite hydrates

The dehydration of uranyl minerals can affect phase structure and stability . Synthetic autunite hydrates, Ca[(UO2) (PO4)](2). xH(2)O, were studied by X-ray powder diffractometry (XRD) and thermogravimetric analysis (TGA) to a ddress ambiguous or contradictory reports in the literature. Structurally, XRD analysis supported the three well-defined phases commonly reported in t he literature, i.e. autunite, metaautunite I, and metaautunite II. In addit ion, 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 accumula tion is tolerated to different degrees among the autunite hydrates. Loss of low temperature water appears to initiate collapse of the interlayer spaci ng from 10 to 9 Angstrom 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 de hydration pattern of autunite is not observed in all the 1:1 uranyl phospha tes, such as chernikovite (H[(UO2)(PO4)]. 4H(2)O).