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3.2 Crystal Chemistry and Phase Transformations

Physical and chemical properties of any materials including those that constitute the Earth depend, under equilibrium conditions, on temperature, pressure, the atomic arrangement, and the activities of all chemical components involved. The arrangement may be periodic with short-range and long range order as in crystalline solids or with short-range order only as in the amorphous state. Crystal chemistry and crystal physics are concerned with such relationships between structures and properties and form the basis for understanding (instead of just describing) the behaviour of matter and for predicting properties under chemical or physical conditions that may not be attainable in the laboratory. On the other hand, crystal structures react to changes in physical and chemical conditions, forming denser packings with increasing pressure, or increasing structural disorder with increasing temperature. They may alter their chemical composition if component activities change. The subdivision of the Earth's mantle into upper mantle, transition zone and lower mantle is due to such transformations, and they play an active role in influencing or even controlling convection.

Transformations between crystal structure types may proceed by completely different mechanisms: they may be continuous (second or higher order), such as an increase of intracrystalline disorder or structural strain with temperature, or they may be reconstructive, where the new phase is formed by a nucleation and growth process and may have only a topotactical relationship to the precursor structure. Displacive phase transitions, on the other hand, always mark the change between two closely related structure types: no major atomic bonds are broken, but individual building blocks such as SiO4 tetrahedra or SiO6 octahedra are kinked relative to each other. Such transitions are generally fast kinetically and therefore often require in-situ studies at elevated pressure and temperature. Although the structural changes are minor, influence of such phase transitions on physical properties (such as electrical conductivity and elasticity) may be drastic, and therefore relevant from the point of view of both material sciences and geology.

A close look at most major mineral groups shows that such effects are nearly ubiquitous. The following section gives an overview of recent work in the field, ranging from problems in the major mineral groups of the mantle and crust to superhard materials, and even archeometrical applications.

Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
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