BÖTTCHER, M.E. & GEHLKEN, P.-L. (1997):
FT-IR spectroscopic characterization of (Ca,Cd)CO3 solid solutions: Compositional and carbon isotope effects.

The pesent study deals with the FT-IR spectra of CaCO3-CdCO3 (calcite-otavite) solid solutions which have been measured to investigate the relation between chemical composition and the wavenumbers of internal and combination modes of the carbonate ion group. In the mid-infrared spectrum of rhombohedral carbonates with space group Rc, three active internal modes are expected which arise from the out-of-plane bending (2), the antisymmetric stretching (3), and the in-plane bending mode (4) of the carbonate ion group (12CO32-). Continous frequency shifts with chemical composition have been measured for several binary and ternary carbonate solid-solution systems. Besides different amounts of foreign cations, natural carbonates generally contain small amounts of 13C in substitution for 12C, and a small satellite band which is shifted to lower wavenumbers with respect to the main 2 band has been attributed to the out-of-plane bending mode of 13CO3 2-. The influence of isotope substitution on absorption band positions is of fundamental interest for the theoretical prediction of stable isotope partitioning.

Besides the results of FT-IR spectroscopic measurements on synthetic Ca12CO3-Cd12CO3 solid solutions, we report here the influence of stable carbon isotope substitution for the pure end members.

The FT-IR spectra of all carbonates display the three expected internal modes, as well as the (1+3) and (1+4) combination modes. As theoretically predicted, no splitting of absorption bands is observed. Additionally, the symmetric stretching mode, 1, is generally observed as a weak absorption band at approximately 1080 cm-1, although it is theoretically forbidden for carbonates with the Rc structure.

As a function of chemical composition , the wavenumbers of the absorption bands of (Ca,Cd)12CO3 solid solutions shift between the end-member values. Increasing substitution of Ca2+ by Cd2+ leads to increasing wavenumbers for 3, 4, (1+3), and (1+4) but decreasing values for 1 and 2. Good linear correlations are observed between chemical compositions and the wavenumbers of the sharp in-plane and out-of-plane bending modes according to Cd12CO3 (mol %) = – 6.3453 x 2 (12CO32-) + 5556.54 (r2 = 0.996) and Cd12CO3 (mol %) = 9.4636 x 4 – 6739.82 (r2 = 0.991). These equations are well suited to quantify the extent of Cd2+ homogeneous incorporation into the calcite lattice with an uncertainty below ± 5 mol % Cd12CO3. The substitution of 12C by 13C leads to shifts of the 2, 3, and (1+3) modes to lower wavenumbers. The measured carbon-isotopic shift coefficients for the internal modes of calcite and otavite agree well with the theoretical values for carbonates with the calcite structure. Accordingly, a mean value of 0.971 ± 0.001 is calculated from the positions of the respective main and satellite out-of-plane bending modes of all investigated carbonates.