Crystal Chemistry of Carnotite in Abandoned Mine Wastes

The crystal chemistry of carnotite (prototype formula: K₂(UO₂)₂(VO₄)₂·3H₂O) occurring in mine wastes collected from Northeastern Arizona was investigated by integrating spectroscopy, electron microscopy, and x-ray diffraction analyses. Raman spectroscopy confirms that the uranyl vanadate phase present in the mine waste is carnotite, rather than the rarer polymorph vandermeerscheite. X-ray diffraction patterns of the carnotite occurring in these mine wastes are in agreement with those reported in the literature for a synthetic analog. Carbon detected in this carnotite was identified as organic carbon inclusions using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analyses. After excluding C and correcting for K-drift from the electron microprobe analyses, the composition of the carnotite was determined as 8.64% K₂O, 0.26% CaO, 61.43% UO₃, 20.26% V₂O₅, 0.38% Fe₂O₃, and 8.23% H₂O. The empirical formula, (K_1.66 Ca_0.043 Al(OH)²⁺ _0.145 Fe(OH)²⁺ _0.044)((U_0.97)O₂)₂((V_1.005)O₄)₂·4H₂O of the studied carnotite, with an atomic ratio 1.9:2:2 for K:U:V, is similar to the that of carnotite (K₂(UO₂)₂(VO₄)₂·3H₂O) reported in the literature. Lattice spacing data determined using selected area electron diffraction (SAED)-TEM suggests: (1) complete amorphization of the carnotite within 120 s of exposure to the electron beam and (2) good agreement of the measured d-spacings for carnotite in the literature. Small Differences between the measured and literature d-spacing values are likely due to the varying degree of hydration between natural and synthetic materials. Such information about the crystal chemistry of carnotite in mine wastes is important for an improved understanding of the occurrence and reactivity of U, V, and other elements in the environment.