Spectroscopic investigations relating to the structural, crystal-chemical and lattice-dynamic properties of (Fe2+,Mn2+,Mg,Ca)3Al2Si3O12 garnet: A review and analysis
Charles A. Geiger, 2004. "Spectroscopic investigations relating to the structural, crystal-chemical and lattice-dynamic properties of (Fe2+,Mn2+,Mg,Ca)3Al2Si3O12 garnet: A review and analysis", Spectroscopic methods in mineralogy, Anton Beran, Eugen Libowitzky
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Garnet is an important phase in various technological applications and in nature as a major rock-forming mineral. The geological occurrence of silicate garnets is widespread and they are stable over an enormous range of rock compositions and pressure and temperature conditions. They are found in low-pressure metamorphic contact aureoles and they occur as complex solid solutions in the Earth's transition zone. Therefore, over the years a large amount of mineralogical, geochemical and mineral physics research has been directed toward garnet. It is, for example, a key mineral in many geochemical trace-element studies concerned with melting in the deep Earth, and in geophysical investigations of the upper mantle and transition zone its physical properties are of importance. In petrologic studies the thermodynamic mixing properties of garnet solid solutions play a central role in many geothermometers and geobarometers. The end-member aluminosilicate garnets [X3Al2Si3O12 with X = Fe2+ (almandine – Al), Mn2+ (spessartine – Sp), Mg (pyrope – Py), and Ca (grossular – Gr)] and their solid solutions (Fig. 1) have received much study regarding their structural, physical, chemical and thermodynamic properties (for a review of the latter see Geiger, 1999). However, there is no comprehensive review article related to their structural, crystal chemical and lattice dynamic properties, and there have been no attempts made to bring together and analyse the rich and diverse literature on spectroscopic investigations. This is the goal of this article.
Various spectroscopic measurements on aluminosilicate garnets covering the full range of the electromagnetic spectrum will be reviewed and an attempt will be made to reach a broad synthesis.
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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.