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.