| Autor: |
Wayne P. Hess, R. A. Mason, S. H. Withers, Robert E. Peale, Richard J. Reeder, Kenneth M. Beck, Evert J. Elzinga |
| Rok vydání: |
2002 |
| Předmět: |
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| Zdroj: |
Geochimica et Cosmochimica Acta. 66:2875-2885 |
| ISSN: |
0016-7037 |
| DOI: |
10.1016/s0016-7037(02)00888-8 |
| Popis: |
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. |
| Databáze: |
OpenAIRE |
| Externí odkaz: |
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