Raman spectroscopic detection of 17O substitution in calcite.

We investigated Raman spectral features of calcite associated with natural oxygen isotope substitution in the CO32- ion. Weak bands were observed at 1075 cm-1 and 1065 cm-1 on the lower-wavenumber side of the main ν1 (A1g) band at 1086 cm-1. Density functional theory (DFT) calculations of isolated CO32- ions predicted isotopic shifts of -10.3 cm-1 for 17O substitution and - 19.8 cm-1 for 18O substitution. The 1065 cm-1 and 1075 cm-1 bands are therefore assigned respectively to the 12C16O16O18O and the 12C16O16O17O isotopologues. Furthermore, we measured the Raman spectrum of cooled calcite, which narrowed the bandwidth and clarified the 12C16O16O17O band at the tail of the main 12C16O3 band considerably. Because of the high spatial resolution of micro-Raman spectroscopy, these results open a path to detailed determination of the three‑oxygen isotope distribution within calcite. Determining the three oxygen isotope (i.e., 16O, 17O, 18O) profiles within calcite will expand our understanding of carbonate formation environments.