IR and FTIR spectroscopic investigations in parts of the quaternary systems CaCO3-MnCO3-MgCO3-FeCO3 and CaCO3 -CdCO3-MnCO3-MgCO3.

The infrared spectroscopic investigations of binary and ternary parts of the quaternary rhombohedral carbonate system CaCO3-MnCO3-MgCO3-FeCO3 (BÖTTCHER et al. 1991, 1992) are extended to the system CdCO3-CaCO3-MgCO3-MnCO3.

Binary and ternary solid solutions from the system CdCO3-CaCO3- MnCO3-MgCO3 have been prepared at 20 ºC (1 atm total pressure) by mixing Me(NO3)2 and (NH4)2CO3 solutions. Complete miscibility is observed in the investigated parts of the system CdCO3-CaCO3- MnCO3 under the chosen conditions.

Infrared spectroscopic measurements have been carried out using specimens with grain sizes less than 2 µm, embedded in KBr. Three active internal modes, 2, 3 and 4, occur in the mid-infrared spectrum of rhombohedral carbonate minerals. The sharp 2 and 4 bands arise from out-of-plane and the in-plane bending mode of the carbonate ion, respectively.

Significant variations of the 2 and 4 positions are observed in these systems due to cationic substitution. Plots of 2 versus 4, introduced by BÖTTCHER et al. (1991, 1992), reflect the chemical compositions of ternary rhombohedral carbonate minerals. Fourier transformed infrared spectroscopic measurements show that the position of the broad 3 band, which arises from the asymmetric-stretching mode of the carbonate ion, should not be used for the determination of chemical composition and may be influenced by the solids formation conditions.