The main aim of this chapter is to introduce spectroscopic methods as complementary techniques to diffraction in the study of the structure of minerals. In terms of the interaction of radiation with matter, diffraction involves a change in direction of the incident radiation, without any change in its energy. The determination of structure by diffraction depends on the periodicity of the structure and therefore produces a long-range or average picture. Any local disorder such as defects, impurities etc. appears in the diffraction pattern as diffuse scattering in the background, and in X-ray structure analysis in which only the Bragg reflections are used, this information is generally not used. In high resolution electron microscopy, the background scattering is automatically added into the Fourier synthesis carried out by the lens, and so the image can contain information about local disorder, although the resolution is limited, as explained in the previous chapter. Spectroscopic methods, on the other hand, provide information about local structure – site symmetry, coordination number, local chemical and crystallographic environment etc. and the methods do not depend on long-range periodicity or crystallinity. In many cases amorphous materials can be studied equally well.

Although there are very many different spectroscopic methods they all work on the same basic principle. Under some conditions, an incident beam of radiation can be absorbed by matter, or alternatively, can cause the emission of radiation from the material.