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3.3 Crystal Chemistry and Structure

The details of the arrangements of the atoms within a solid determine its physical, mechanical and chemical properties. Similarly, the pressures and temperatures at which phase transitions or changes in structure occur in minerals are also dependent upon details of, and changes in, the local structure of the mineral. Therefore, in order to develop an understanding of the structure of the deep Earth and the processes that occur within it, one must have a clear understanding of the structures of individual minerals on the atomic scale. Information on crystal structures can be derived from a number of experimental techniques that probe a range of length scales within a mineral. Diffraction methods, including X-ray powder and single-crystal and neutron powder diffraction yield information on the average structure of a mineral on the length scale of several hundred Ångstrom (1Å = 10-10 m). As illustrated by the example of CaAl4Si2O11 described below, the determination of the average structure may not complete the whole story, but may indicate that some shorter-range disordering occurs within the structure. At the shortest length scales (1 to 10Å) a number of probes are available that allow the local environment of specific individual atoms to be characterized. Thus, optical absorption spectroscopy provides critical information on the electronic structure of transition metal atoms within crystals which enables the details of the distortion of the coordination environment of the metal to be determined. It is clear from the work on transition-metal silicates performed in the Geoinstitut over the past few years that the influence of electronic structure in the form of "crystal field effects" is important in determining the high-pressure behaviour of minerals. In particular, the examples of clinopyroxenes and gillespites discussed below illustrate the dominant influence of crystal-field effects on the phase transition behaviour of silicates. Mössbauer spectroscopy is also a local probe, specifically of the major transition metal component of most minerals, iron. The examples of Mössbauer investigations reported here cover minerals that occur in the Earth's crust such as clinopyroxenes, mantle garnets and perovskites. Intermediate length scales can be probed by electron diffraction and imaging of samples within the electron microscope and, as illustrated by the study of the CaTiO3-CaFeO2.5 perovskite-based system, a complete characterization of the complex processes occurring within a material can often only be made through an integration of the results of all of these structural probes at different length scales.

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