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3.2 Crystal Chemistry and Phase Transformations

Crystal structures and their transformations as a response to changes in pressure and temperature provide a clue to the understanding of the structure of the Earth's interior, the equilibrium states of matter at these depths and the dynamics of processes. The major seismic discontinuities in the Earth's mantle (which originate from sudden changes in elasticity and density), but also fairly abrupt changes in electrical conductivity, are the results of such transformations, leading to denser atomic arrangements with increasing pressure and to very different chemical and physical properties. For instance, we have shown in recent years that the crystal chemical role of Fe (the most important transition metal in the Earth) changes dramatically with pressure: in the most abundant low-pressure silicates such as olivine, Fe3+ is an incompatible element, whereas Fe2+ is not. This means that even under oxidizing conditions, ferric iron contents will be low. Starting from the transition zone, however, where high-pressure polymorphs of the olivine composition (wadsleyite and ringwoodite) become stable, and continuing into the lower mantle that is predominantly composed of silicate perovskite, Fe3+ becomes a compatible element, and Fe2+ might even disproportionate into Fe3+ and Fe metal. The study of the crystal chemistry of mantle phases (or their low-pressure analogs) and the nature of their phase transitions therefore forms the basis for our understanding of their chemical and physical behavior at depths. Implications of such studies also lead us into the more general field of material sciences, because the systematics of high-pressure phase transitions allows us to predict how materials in general - not only those we expect at depth in the Earth - develop novel properties at pressure.

Many of the projects described below are tied into the EC-Network on "Mineral Transformations" that fosters close collaboration with material scientists, physicists and chemists on a European scale.

Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Deutschland
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