CHARACTERIZATION OF SYNTHETIC BACO3-SRCO3 (WITHERITE-STRONTIANITE) SOLID-SOLUTIONS BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

The FTIR spectra of 15 synthetic witherite-strontianite ((BaCO3)-C-12 - (SrCO3)-C-12) solid-solutions containing about 1.1 atom% C-13, are i nvestigated at ambient temperature and atmospheric pressure for the in fluence of cationic substitution on the wavenumbers of infrared-active modes of the carbonate ion group between 4000 and 400 cm(-1). Homogen eous carbonate solid-solutions, covering the compositional range betwe en 8 and 86 mol% strontium, are grown at 25 degrees C by counter-diffu sion through a porous silica-gel transport medium, and the chemical co mpositions are determined by electron microprobe. As a function of the carbonates chemical composition, an increase in frequency of the fund amental (v1, v2, v3, v4) and combination modes ((v1 + v3), (v1 + v4), (v3 + v4)) with increasing Sr substitution is observed. Nearly linear variations of the frequencies between the end-member values as a funct ion of composition are observed. The FTIR spectrum of (BaCO3)-C-13 con taining about 1 atom% C-12 is measured, too. Due to the substitution o f C-12 by C-13, the wavenumbers of most of the infrared-active modes a re shifted to lower values, in agreement with theoretical calculations . Measured isotope shift coefficients (vi((CO32-)-C-13)/(vi((CO32-)-C- 12)) are 1.000 (v1), 0.969 (v2), 0.976 (v3), 0.999 (v4), 0.984 (v1 + v 3), 0.999 (v1 + v4) and 0.980 (v3 + v4). The isotope shift coefficient observed for v2 of the nearly decoupled carbonate ions containing the respective minor carbon isotope is 0.961.