Crystal Chemistry of Carnotite in Abandoned Mine Wastes.

Eric Peterson,Angelica Benavidez,Sumant Avasarala,José M. Cerrato,Adrian J. Brearley,Michael Spilde,Ying-Bing Jiang

doi:10.3390/min10100883

Abstract

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, (K1.66 Ca0.043 Al(OH)2+0.145 Fe(OH)2+0.044)((U0.97)O2)2((V1.005)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.

Highlights

Uranium (U) mining operations during the 1900s resulted in thousands of abandoned uranium mines in the United States
The Raman spectrum of the uranyl vanadate specimen obtained from the Blue Gap Tachee (BGT) mine waste The Raman spectrum of the uranyl vanadate specimen obtained from the BGT mine waste sample sample is shown in Figure 1, compared with spectra obtained from a carnotite from is shown in Figure 1, compared with spectra obtained from a carnotite from the University of New Mexico (UNM)
We showed by Raman spectroscopy and electron diffraction analysis that the uranyl vanadate at the Blue gap Tachee/Claim 28 mine site was carnotite

Summary

Introduction

Uranium (U) mining operations during the 1900s resulted in thousands of abandoned uranium mines in the United States. More than 4500 abandoned U mine waste sites have been identified in the Western United States, of which more than 500 are located on the Navajo Nation [1,2,3,4]. Gap/Tachee Claim 28 mine waste site in Northeastern Arizona was abandoned after mining operations. Despite the reclamation, elevated concentrations of U and vanadium (V) have been measured in the water sources proximate to the Claim. Dissolution of uranyl vanadate (U-V) minerals found at this site was identified as the source of these elevated concentrations [3,5,6]. The reactivity and occurrence of these uranyl vanadates have been studied in detail, understanding of their crystal chemistry is still limited

Objectives

Methods

Results

Conclusion