IR spectroscopic characterisation of hydrous species in minerals
Published:January 01, 2004
Hydrogen is a major, a minor, and a trace constituent of a broad variety of minerals in the Earth's lithosphere. Usually bonded to oxygen in the structures of minerals (in the form of OH- and H2O, rarely also as H3O+, and groups) it contributes to the formation of stoichiometric hydrates and hydroxides, many of them with economic importance, such as zeolites, gypsum CaSO4 · 2H2O, manganite MnO(OH) etc. Even though these stoichiometric hydrous phases are most common in the upper crust, they also persist to greater depths in subduction zones and may be responsible for recycling of water (Pawley, 1994) down to the mantle region (e.g. lawsonite, talc, K-amphiboles etc.). Because of the enormous volume of the Earth's mantle, nominally anhydrous minerals under the high P/T conditions of the Earth's mantle that contain hydrogen only as minor or trace constituents, play an important role for the water budget of the Earth (Beran, 1999 still a controversy as to whether the mantle is rather enriched (Thompson, 1992) or depleted (Dixon et al., 2002) in hydrogen through the activity of subduction zones.
Even as a trace constituent, hydrogen has a strong influence on chemical and physical properties of mineral phases (Mackwell et al., 1985) and thus deserves thorough investigation. It may be responsible for dramatic changes of mechanical properties, e.g. in the case of hydrolytic weakening of quartz (Griggs, 1967; Kronenberg et al., 1986), it has a major impact on the formation of melts and their rheology (Hirth & Kohlstedt, 1996),
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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.