EXAFS study of rare-earth element coordination in calcite |
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Authors: | E.J. ElzingaR.J. Reeder S.H. WithersR.E. Peale R.A. MasonK.M. Beck W.P. Hess |
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Affiliation: | 1 Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY 11794-2100, USA 2 Department of Physics, University of Central Florida, Orlando, FL 32816-2385, USA 3 Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada, A1B 3X5 4 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, USA |
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Abstract: | Extended X-ray absorption fine-structure (EXAFS) spectroscopy is used to characterize the local coordination of selected rare-earth elements (Nd3+, Sm3+, Dy3+, Yb3+) coprecipitated with calcite in minor concentrations from room-temperature aqueous solutions. Fitting results confirm substitution in the Ca site, but first-shell Nd-O and Sm-O distances are longer than the Ca-O distance in calcite and longer than what is consistent with ionic radii sums for sixfold coordination in the octahedral Ca site. In contrast, first-shell Dy-O and Yb-O distances are shorter than the Ca-O distance and are consistent with ionic radii sums for sixfold coordination. Comparison of Nd-O and Sm-O bond lengths with those in lanthanide sesquioxides and with ionic radii trends across the lanthanide series suggests that Nd3+ and Sm3+ have sevenfold coordination in a modified Ca site in calcite. This would require some disruption of the local structure, with an expected decrease in stability, and possibly a different charge compensation mechanism between Nd and Sm vs. Yb and Dy. A possible explanation for the increased coordination for the larger rare-earth elements involves bidentate ligation from a CO3 group. Because trivalent actinides such as Am3+ and Cm3+ have ionic radii similar to Nd3+, their incorporation in calcite may result in a similar defect structure. |
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