Synthesis of a complete BaCO3 – SrCO3 (witherite – strontianite) solid-solution series from aqueous solutions at 25 °C: Characterization of Ba-Sr and 13C-12C substitution by FTIR spectroscopy.

The FTIR spectra of orthorhombic witherite-strontianite solid-solutions are measured at ambient temperature and atmospheric pressure between 4000 and 400 cm-1 for the influence of cationic substitution on the wave-numbers of infrared-active modes of the carbonate ion group. These relationships have hitherto only been investigated for rhombohedral carbonate solid-solutions (BÖTTCHER & GEHLKEN 1995, BÖTTCHER et al. 1992).

A complete set of homogeneous Ba12CO3 – Sr12 CO3 solid-solutions (~ 1 atom% 13C) is grown at 25 ºC by counter-diffusion through a porous silica-gel transport medium by a method described earlier (PRIETO et al. 1989, PUTNIS et al. 1992). The chemical and carbon isotopic compositions are determined by electron microprobe and C-irMS, respectively. The carbonates are confirmed to be members from the witherite-strontianite mineral series by powder XRD, and additionally characterized by back scattering electron imaging. High supersaturations are calculated for the aqueous solutions in the diffusion column at nucleation time. Due to the small differences of the end-member solubility products and the high precipitation rates, the aqueous and solid-solutions virtually show the same strontium activity and mole fractions (Xsr = Sr/(Sr+Ba)), respectively).

The carbonate solid-solutions generally display the fundamental modes 1 , 2, 3 and 4, and the combination modes (1+ 3), (1+ 4) and (3+ 4). Additionally, a complex combination mode is observed near 2570 cm-1. 1, 2, 3 and 4 arise from the symmetric stretching, the out-of-plane bending, the asymmetric stretching and the in-plane bending mode of the carbonate ion group, respectively. Splitting of the 4 mode is only observed for Xsr values above 0.6. As a function of the carbonates chemical composition, significant shifts of the wave-numbers of the fundamental and combination modes are observed. The relationships are well described by: = A + B x Xsr.

The FTIR spectrum of Ba13CO3 is measured, too. Due to the substitution of 12C by 13C, most of the infrared-active modes are shifted to lower wave-numbers in agreement with theoretical predictions (GOLYSHEV et al. 1981). For pure witherite, the experimentally observed carbon isotopic-shift coefficients ( (13CO32-) / (12CO32-)) are 1.000 (1), 0.969 (2), 0.976 ( 3), 0.999 (4), 0.984 ( 1+ 3), 0.999 ( 1+ 4) and 0.980 (3 + 4). It should be noted, however, that the positions of small satellite bands near the main 2 absorption maximum, caused by vibrations of the carbonate group containing the minor carbon isotope (STERZEL 1969), yield isotopic-shift coefficients of 0.981 and 0.958 for Ba12CO3 and Ba13CO3, respectively. These differences are probably caused by a coupling between neighbouring anions in the orthorhombic carbonate lattice (STERZEL 1969)..

Since stable isotope fractionation in the system witherite-strontianite is mainly influenced by the internal vibrations of the carbonate ion group (GOLYSHEV et al. 1981), the observed continuos frequency-shifts of the internal modes in the system BaCO3 – SrCO3 lead to the conclusion that the magnitude of carbon and oxygen isotope fractionation involving SrXBa(1-X)CO3 solid-solutions may be linearely interpolated from the end-member relations.