The Characterization of Soil Minerals by Infrared Spectroscopy

The infrared spectrum of a mineral is a characteristic feature, which permits the identification of mineral species. The absorption bands arise from vibrations of the atoms or ions in the structure, and the frequencies of vibrations are dependent on the mass of the atoms, the restraining forces of the bonds, and the geometry of the structure. As a result, the spectrum of a mineral is sensitive to isomorphous replacements in its structure, as these affect both bond strengths and atom masses. The symmetry and regularity of a structure play an important part in determining the intensity and frequency of its vibrations, so that the infrared spectrum is often a sensitive indicator of the degree of order of a crystalline mineral. Amorphous structures absorb infrared radiation as strongly as crystalline structures, although their absorption bands are broader and show fewer distinctive features. Nevertheless, infrared spectroscopy is one of the few techniques which can yield information on the structure and composition of amorphous phases.