EXAFS study of rare-earth element coordination in calcite

    Authors

    E. J. Elzinga; R. J. Reeder; S. H. Withers; R. E. Peale; R. A. Mason; K. M. Beck;W. P. Hess

    Abbreviated Journal Title

    Geochim. Cosmochim. Acta

    Keywords

    X-RAY; INORGANIC COMPLEXES; SINGLE-CRYSTALS; TRACE-ELEMENTS; SOLID-SOLUTION; SURFACE; XAFS; 25-DEGREES-C; PARAMETERS; CARBONATE; Geochemistry & Geophysics

    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 (C) 2002 Elsevier Science Ltd.

    Journal Title

    Geochimica Et Cosmochimica Acta

    Volume

    66

    Issue/Number

    16

    Publication Date

    1-1-2002

    Document Type

    Article

    Language

    English

    First Page

    2875

    Last Page

    2885

    WOS Identifier

    WOS:000177376200006

    ISSN

    0016-7037

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