Raman spectroscopy of the eight natural carbonate minerals of calcite structure

To date, only five natural carbonate minerals of calcite structure have been studied by Raman spectroscopy. These include calcite (CaCO3), magnesite (MgCO3), siderite (FeCO3), smithsonite (ZnCO3), and rhodochrosite (MnCO3). Thus far, only synthetic compounds of otavite (CdCO3), spherocobaltite (CoCO3), and gaspeite (NiCO3) have been investigated by Raman spectroscopy. However, the Raman spectra of natural otavite, spherocobaltite, and gaspeite have yet to be interpreted and compared with the Raman spectra of the other five natural carbonate minerals of calcite structure. This work has been undertaken to fill this gap and provide a comparison and interpretation of Raman spectra representative of all the eight natural carbonate minerals of calcite structure. The data here show that the carbonate E-g (T) phonon shifts are due to influences from the nearest neighbor distance; that is, M-O, and different ionic radii of the divalent metal cation, as shown graphically by a strong correlation (r(2) = 0.87 and 0.91, respectively). Using this graphical approach, we have developed a Raman spectroscopic model based on the equation, y = -2.067x + 356.2 (+/- 5 pm) to calculate the ionic radii of the divalent metal cation present within the mineral and hence affording the identification and discrimination of calcite-group minerals based on the band position of the E-g (T) mode.