A more complicated case illustrating the difference between infrared (IR) ab-
sorption spectroscopy and Raman scattering spectroscopy is provided by the
CO2−3 groups in crystals of carbonates. In crystals with molecular ions (e.g., car-
bonates, borates, sulfates, and others) the molecular groups have strong covalent
bonding and are only weakly disturbed by neighboring cations. Distortions of the
CO 2−3 group gives rise to high-frequency vibrations within the molecule (so-called
internal vibrations). These vibrations have energies that are very characteristic of the
molecule and are easy to identify. There are also low-frequency vibrations owing
to the interaction of the molecule with other ions. These external modes consist
not of distortions of the molecular group but rather of librations (rotations) or
translations of the whole group with respect to the other ions of the crystal. The
external modes are structure specific but are more difficult to attribute.
Minerals: Their Constitution and Origin
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