Non-destructive estimation of the cation composition of natural carbonates by micro-Raman spectroscopy

Carbonates play a crucial role in the water and carbon cycles of both geochemical and cosmochemical environments. As carbonates do not exist homogeneously with other minerals in rocks and sands of various sizes, an analytical method that simultaneously satisfies non-destructivity and high spatial resolution has been desired. Further, the ability of semi-quantitative analysis with carbonates-selectivity and without any pre-treatments is added, for its applicability would be extended to remote sensing for deep sea and outer spaces. Here, we focused on the application of micro-Raman spectroscopy, where the vibrational wavenumbers of the translational (T) and librational (L) modes of carbonates are sensitively related to their cation composition. By comparing the semi-quantitative information obtained by X-ray fluorescence spectroscopy, it was found that these vibrational wavenumbers are approximately linearly related to the cation composition. Consequently, a conversion matrix was proposed to estimate the cation composition from the T and L mode vibrational wavenumbers. This method is universally applicable to any cation composition in carbonates, with no background information on the analyte required. To improve the accuracy, conversion matrices were further optimized to three solid-solution series of carbonates. It is worth noting that the proposed conversion matrices are free from matrix effects and do not depend on the total amount of carbonate in a sample. Therefore, the proposed method provides a useful analytical basis for remote sensing of the cation composition of carbonates, both in terrestrial and extra-terrestrial environments.