FT-Raman and infrared spectroscopic study of aragonite-strontianite (CaxSr1-xCO3) solid solution

Synthetic aragonite-strontianite solid-solution samples have been studied using dispersive IR and FT-Raman spectroscopy. In addition to the end-members, nine samples over a range of composition from Ca0.9Sr0.1CO3 to Ca0.1Sr0.9CO3 were analysed. Carbonate anion internal modes are examined in detail by means of band-shape analysis and component fitting procedures. Positional disorder induced by the random cationic substitution results in strong increase of the halfwidth in several vibrational bands. Results obtained for the doubly degenerate modes (antisymmetric stretching and bending, nu(3) and nu(4)) reveal the presence of three components both in the TR antisymmetric stretching band as well as in the Raman antisymmetric bending band. These observations are interpreted in terms of an overtone 2 nu(4) in possible Fermi resonance with the corresponding nu(3) fundamental in the IR spectra, and the presence of Davidov (factor group) splitting in the FT-Raman nu(4) band. Lattice modes in the FT-Raman spectra demonstrate weaker cohesion between the cation/carbonate/cation layers in aragonite (synth) than in strontianite (synth). (C) 1997 Elsevier Science B.V.