An understanding of chemical bonding in a system can be gained through calculations based on the theoretical approaches outlined in the previous chapter, or through experimentation. In a much more limited way, it is also possible to gain some understanding of the bonding in a system by a “phenomenological” application of (qualitative) theory given certain properties of the system (e.g., chemical composition, crystal or molecular structure, magnetic and electrical behavior, etc.). Ideally these approaches should be combined so as to gain a unified understanding of the bonding in a particular system. It is very important that the results of quantum-mechanical calculations are compared with experimental data so as to assess their validity. Conversely, the results of calculations may be used in the interpretation of the data from experiments. In this chapter, the wide range of experimental methods that can provide information on chemical bonding in geochemical systems is reviewed. Following a very brief summary of the principles of each technique, some examples are given of its application to minerals (or other systems of geochemical interest, such as melts, glasses, or aqueous solutions). The objective is to draw attention to techniques of importance and to show their relevance to bonding studies and their relationships both to quantum-mechanical calculations and to other experimental methods. No attempt is made to explain the theoretical background of these techniques fully or the practical problems involved in their application. Indeed, each of them has spawned a substantial literature, including books and review articles, some of which are cited here for the reader requiring further details. The experimental methods to be discussed have been divided into five major categories—diffraction effects, electron and x-ray spectroscopies, optical (uv-visible-near-ir) spectroscopy, vibrational spectroscopy, and nuclear spectroscopy. A number of techniques are also discussed in the sixth category—”other methods.” Nevertheless, the range of techniques discussed is very far from complete, and a fuller listing is given in Appendix B. This Appendix also serves to provide some useful references on each technique and a key to the numerous acronyms and abbreviations used throughout the literature to refer to these techniques.
Roby-Gould bond indices as a tool for understanding chemical bonding from a mathematical and quantum mechanical perspective