Raman spectroscopic detection of ¹⁷O substitution in calcite

We investigated Raman spectral features of calcite associated with natural oxygen isotope substitution in the CO₃^2- ion. Weak bands were observed at 1075 cm^-1 and 1065 cm^-1 on the lower-wavenumber side of the main ν₁ (A_1g) band at 1086 cm^-1. Density functional theory (DFT) calculations of isolated CO₃^2- 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.