Characterization of cationic substitution in anhydrous carbonate solid-solutions by FTIR and raman spectroscopy.

Infrared and Raman spectroscopy are standard methods for the detection of carbonate minerals in mineralogical and material science studies. However, identification of carbonates may be rendered difficult by the substitution of cations. Therefore, a systematic study using Fourier Transform Infrared (FTIR) and Raman spectroscopy has been carried out on a variety of anhydrous carbonate solid-solutions for the influence of cationic substitution on the wavenumbers of infrared and Raman active internal and combination modes of the carbonate ion group.

The investigated binary rhombohedral carbonate solid-solution systems are: CaCO3 – CaMg(CO3)2 – MgCO3 (calcite-dolomite-magnesite), CaCO3 – MnCO3 (calcite-rhodochrosite), CaCO3 – FeCO3 (calcite-siderite), MgCO3 – FeCO3, CaCO3 – CdCO3 (calcite-otavite), CdCO3 – MnCO3, and MnCO3 – ZnCO3 (rhodochrosite – smithsonite). The orthorhombic BaCO3 – SrCO3 (witherite – strontianite) system was considered, too. The influence of different synthesis conditions on the vibrational modes was investigated for the systems CaCO3 – MgCO3 and CaCO3 – MnCO3. Additionally, the results from synthetic materials were compared to natural mineral samples. Cationic substitution in ternary solid-solutions, as well as the effects of cationic ordering were investigated in the systems CaCO3 – MgCO3 – FeCO3, CaCO3 – MgCO3 – MnCO3, and CaCO3 – MnCO3 – CdCO3.

In all systems significant shifts in frequencies of fundamental and combination modes („one-mode behaviour“) were observed as a function of chemical composition. In most systems deviation of the frequency variations from linear trends connecting the end-member values indicate non-ideal mixing and may in some cases be related to disordering of the carbonate ion group. Ideal mixing is deduced for the synthetic systems CaCO3 – CdCO3 and BaCO3 – SrCO3.