Vibrational and First-Principles Analyses of Aragonite, Calcite, and Vaterite CaCO₃ Prepared from ¹³CO₂ Isotope

To clarify the isotopic fractionation of CaCO₃, Ca¹²CO₃ and Ca¹³CO₃ were synthesized from ¹²CO₂ and ¹³CO₂, respectively, and analyzed using infrared absorption spectroscopy. During the synthesis, calcite, aragonite, and vaterite were successfully separated. Many different vaterite structures have been proposed, and the structure of the synthesized vaterite was identified using X-ray diffraction. The infrared absorption data for calcite, aragonite, and vaterite composed of 100% ¹³C were obtained for the first time, providing precise data on the isotopic shift of the reference vibration of the CO₃ ^2- ion in CaCO₃. Vibrational analyses using first-principles calculations reproduced the experimentally measured isotope shifts to a certain degree. The reduced isotopic partition function ratios for each vaterite structure, which were not experimentally determined, were theoretically obtained via vibrational analysis of each vaterite structure.