XAFS study of the coordination and local relaxation around Co2+, Zn2+, Pb2+, and Ba2+ trace elements

Analysis of divalent Co, Zn, Pb, and Ba XAFS spectra of synthetic and natur al calcite samples containing trace concentrations of these heavy metals co nfirms their substitution in the unique Ca site in octahedral coordination with varying degrees of local distortion. The existence of each trace metal at the single site in the bulk crystal is significant in view of previous studies showing that these trace elements are incorporated differentially a t multiple, structurally distinct surface sites occurring in nonequivalent growth steps on calcite surfaces. The octahedral coordination for Ba is par ticularly noteworthy because of its large size (35% larger than host Ca) an d the fact that it rarely exists as a major constituent with such a low coo rdination number. Analysis of the local distortion and relaxation around the impurities shows a nearly complete contraction of the structure around Co2+ and Zn2+. The C o-O and Zn-O first-shell distances are only slightly longer than in CoCO3 a nd ZnCO3, with a site compliance of similar to 80-90%. Displacements of hig her shells relative to those in calcite decrease rapidly, but irregularly, over a short distance, and the relaxation may be largely confined within 6- 7 Angstrom of the impurity. The dilation around the large Pb2+ and Ba2+ ion s in calcite also shows a high degree of site compliance (85-90%). Relaxati on around Pb and Ba also appears to be restricted, but extending further fo r Ba than for Pb. The limited observations suggest that compliance of the o ctahedral site in calcite is larger than for the cation site in the rocksal t structure. The high compliance for the metal site in calcite may reflect the corner-sharing topology of the structure and is also probably the reaso n that calcite shows such a wide range of isovalent substitutions in nature . The findings also provide a direct indication of the local strain associa ted with a dilute substitutional solid solution.