Bond lengths differences between the mollusk-made and geological calcium carbonate

We used high-resolution neutron powder diffraction technique in order to accurately measure the atomic positions and bond lengths in calcium carbonates of biogenic (mollusk-made) and geological origin. As a result, in biogenic calcium carbonate we identified atomic bonds, first of all the CO bonds and some OO bonds, which obey significant modification (about 1%) with respect to those in geological calcium carbonate. Bond length changes are presumably due to the organic/inorganic interactions in natural bio-composites. Generally, the effect is more pronounced for aragonite, which is structurally more flexible (nine unfixed parameters in atomic positions) than calcite (one parameter of this kind only). The observed bond modifications can be a source of the reported changes in the frequencies of normal vibrations of the carbonate groups measured by Raman or Fourier-transform infrared spectroscopy (FTIR) techniques.