POLARIZED RAMAN SCATTERING OF DOLOMITE AND SPECTRAL DISCRIMINATION BETWEEN MAGNESITE AND CALCITE

Despite being an advantageous technique, the application of Raman spectroscopy to discriminate between dolomite and magnesite, and to a lesser extent calcite, has by and large, resulted in erroneous carbonate mineral identification. This has been facilitated by some researchers noting that Raman spectroscopy is not useful for distinguishing calcite from dolomite because these two minerals have, except for the ordering of Ca²⁺ and Mg²⁺ ions in dolomite, the same crystal structure and thus very similar spectra [e.g., 1]. Some researchers, however, have observed the presence of one to three additional bands, in the Raman spectra of dolomite, at different relative wavenumbers, by comparison with calcite and magnesite [e.g., 2]. To date, it is not clear what, if any, spectral differences exist between dolomite and calcite or magnesite. Therefore, in this work, a systematic experimental approach has been undertaken to elucidate the differences between the Raman spectra of calcite (a calcite-group mineral) and dolomite (a dolomite-group mineral) and further, polarized Raman scattering of natural single-crystal optical quality dolomite, via selection rules to understand better the assignment of the phonon modes arising from the center of the Brillouin zone, Γ point. Here, we demonstrate that nuclear site group analysis and polarized Raman scattering furnish successful discrimination between these two carbonate mineral groups. Hence, upon application, these bands will eliminate further misidentification of these important carbonate mineral groups. Our work shows that Raman spectra acquired from dolomite contain three additional bands at 273, 335, and 880 cm^-1, which can be assigned as A_g symmetry in character after polarization Raman scattering analyses via selection rules.

References

[1] White, W.B., (2006). Identification of cave minerals by Raman spectroscopy: new technology for non-destructive analysis. International Journal of Speleology, 35(2), p.6.

[2] Valenzano, L., Noel, Y., Orlando, R., Zicovich-Wilson, C. M., Ferrero, M., & Dovesi, R. (2007). Ab initio vibrational spectra and dielectric properties of carbonates: magnesite, calcite and dolomite. Theoretical Chemistry Accounts, 117, 991-1000.