Exploring the Raman oxygen isotope signatures of calcite and vaterite

Oxygen isotope tracers have been increasingly used to differentiate between solid-state and fluid-mediated mineral transformation pathways (e.g., Julia et al. 2023). When ¹⁸O enrichment within oxyanion bearing minerals is analysed using Raman spectroscopy, the kinetically hindered formation of different isotopologues can also provide an in-situ timer for these processes (King et al. 2014). However, at present we assume that each isotopologue band in the vibrational spectrum has an equivalent intensity when present at the same concentration within the crystal structure. Here we test this hypothesis by exploring the vibrational spectra of two important oxyanion-bearing minerals, calcite and vaterite. These are polymorphs of CaCO₃ and reflect a metastable, transition phase and the thermodynamically most stable mineral expected in many natural systems found at the Earth’s surface.
Here we have used a joint experimental and theoretical approach to demonstrate that isotopic substitution changes both band positions and band intensities to different extents, depending on the vibrational spectroscopy method used and the bands examined. Density functional theory simulations (King et al. 2022) show that for calcite, the most intense Raman bands, the υ1 symmetrical stretching, related to individual isotopologues are found to have very similar intensities and are not affected by changes in isotopologue distribution within the material. Splitting of some bands due to changes in symmetry correlate to observed effects in experimentally produced ¹⁸O enriched calcite. In contrast, vaterite vibrational bands were found to change more extensively upon isotope substitution, thus they can only be used to evaluate relative changes in the ¹⁸O concentration within the material. These results are expected to contribute to a deeper und less ambiguous understanding of evaluating isotopic enrichment effects in the vibrational spectra of calcium carbonates.
Julia et al. 2023 Chemical Geology, 621, 121364.
King et al. 2014 Crystal Growth & Design, 14, pp. 3910.
King et al. 2022 Crystals, 13, 48.