EXAFS study of rare-earth element coordination in calcite

Abstract

Abstract-Extended X-ray absorption fine-structure (EXAFS) spectroscopy is used to characterize the local coordination of selected rare-earth elements (Nd 3ϩ , Sm 3ϩ , Dy 3ϩ , Yb 3ϩ ) 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 Nd 3ϩ and Sm 3ϩ 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 CO 3 group. Because trivalent actinides such as Am 3ϩ and Cm 3ϩ have ionic radii similar to Nd 3ϩ , their incorporation in calcite may result in a similar defect structure.

Determination of the host phase of rare earth elements in natural carbonate using X-ray absorption near-edge structure

“…Tanaka et al (2004) observed a kink of REE partitioning pattern between calcite and aqueous solution at Sm possibly due to such a change of local coordination structure. The local coordination structure of Y and Ho reported by Tanaka et al (2008) was consistent with those of Dy and Yb by Elzinga et al (2002). It is likely that the local structure of Y in calcite is similar to those of heavy REEs from trivalent Eu to Lu.…”

“…In a previous study (Tanaka et al, 2008), we found a peculiar white line of Y K-edge XANES spectrum for synthetic Y-doped calcite. However, such a feature was not observed in the L III -edge XANES spectra of other REEs when they were coprecipitated with calcite (Elzinga et al, 2002;Tanaka et al, 2008). In addition, Y concentration in natural samples is high enough to measure the XANES spectra, although it is difficult to measure extended X-ray absorption fine structure (EXAFS) spectra of natural samples.…”

“…Numerous studies dealing with the estimation of REE pattern in ancient oceans by carbonate are based on the premise that REEs are incorporated into the carbonate phase of natural samples, but not in other minor phases such as Fe-Mn oxides and phosphate in limestone. This assumption is probably valid since Elzinga et al (2002) used X-ray absorption fine structure (XAFS) spectroscopy to confirm that REE was substituted for Ca site in "synthetic" calcite. In order to apply experimental partition coefficients properly to natural samples, it must be verified that the incorporation mechanism of REE into carbonate in natural environments is the same as that in laboratory partitioning experiments (Tanaka et al, 2004;Tanaka and Kawabe, 2006).…”

“…Elzinga et al (2002) pointed out the difference of local coordination structure in calcite between light REE and heavy REE using EXAFS spectroscopy. In short, Nd and Sm have seven-fold coordination, whereas the coordination number of Dy and Yb is six.…”

Experimental study of rare earth element partitioning between calcite and sodium chloride solution at room temperature and pressure

“…On the partition coefficient versus ionic radius diagram (PC-IR diagram) for the phenocryst-magma pairs, it is generally known that a parabolic curve which shows a maximum of partition coefficients at the ionic radius near the optimum size for major ion site of host mineral can be drawn for trace ions of the same charge (e.g., Onuma et al, 1968;Möller, 1988;Blundy and Wood, 2003). The studies using extend Xray absorption fine structure (EXAFS) spectroscopy by Elzinga et al (2002) and Withers et al (2003) showed that the investigated lanthanides occupy a Ca site in calcite structure. Combining such observation and the above hypothesis, it is expected that the REE partition coefficients for calcite-aqueous solution system demonstrate parabolic shape on the partition coefficient versus ionic radius diagram.…”