Title

Exafs Study Of Rare-Earth Element Coordination In Calcite

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. Copyright © 2002 Elsevier Science Ltd.

Publication Date

8-1-2002

Publication Title

Geochimica et Cosmochimica Acta

Volume

66

Issue

16

Number of Pages

2875-2885

Document Type

Article

Personal Identifier

scopus

DOI Link

https://doi.org/10.1016/S0016-7037(02)00888-8

Socpus ID

0036690762 (Scopus)

Source API URL

https://api.elsevier.com/content/abstract/scopus_id/0036690762

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