Spectroscopic methods in mineralogy
Spectroscopic methods provide information about the local structure of minerals. The methods do not depend on long-range periodicity or crystallinity. The geometric arrangement of atoms in a mineral phase is only one aspect of its constitution. Its vibrational characteristic, electronic structure and magnetic properties are of greatest importance when we consider the behaviour of minerals in dynamic processes. The characterisation of the structural and physico-chemical properties of a mineral requires the application of several complementary spectroscopic techniques. However, it is one of the main aims of this School to demonstrate that different spectroscopic methods work on the same basic principles. Spectroscopic techniques represent an extremely rapidly evolving area of mineralogy and many recent research efforts are similar to those in materials science, solid state physics and chemistry. Applications to different materials of geoscientific relevance have expanded by the development of microspectroscopic techniques and by in situ measurements at low- to high-temperature and high-pressure conditions.
NMR has been used very widely for the last 25 years to study the structures of glasses. Chemists and materials scientists need to understand how the structure and properties of glasses are related in order to improve the properties and processing of technologically useful glasses. Earth scientists use glasses as analogues for silicate melts because of the experimental difficulties associated with making spectroscopic and other measurements on molten silicates at extremely high temperatures. An array of different NMR experiments has been applied to glasses, and novel information has been obtained on structures and dynamic processes over a variety of length and time scales. It should be noted at the outset that glass structure represents the structure of a metastable melt at Tg, the glass transition temperature, i.e. usually several hundred degrees below the liquidus. The detailed information on glass structure available from NMR studies, especially where Tg is modified by changing the thermal history of the glass, must therefore be integrated with other spectroscopic and thermodynamic measurements of melts at high temperature to obtain the most complete understanding of silicate melt structure.
Several excellent reviews of the application of NMR to glasses have been published, e.g. Eckert (1992), MacKenzie & Smith (2002), Stebbins (1995a, 2001), Zwanziger (1998), and the subject is now far too large to be covered comprehensively in a single review paper. In this contribution I will therefore only discuss the structures and properties of silicate glasses, and will focus on a few key examples of the type of information which is now available.