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Crystal Chemistry is the aspect of the research pursued at the Bayerisches Geoinstitut that has as its aim an understanding of the properties of minerals through the determination of their structure and bonding. However this end cannot be achieved by studying minerals alone. Understanding must derive from a comparison of a number of similarly-structured materials, and concepts of physical behaviour must be verified against analogues. Such an approach has long been the basic philosophy of this institute in which simple experimental systems are often used as analogues to develop understanding of more complex natural systems. This approach is exemplified here by the experimental study of Fe³⁺ substitution into perovskites and the determination of the phase transition behaviour of akermanite. This approach has been extended this year by the study of completely synthetic systems a further step removed from minerals. Thus, for example, our understanding of the stability of the mineral wadsleyite, a major component of the transition zone, can be tested and developed by studying the isotypic spinelloids discovered this year in the system magnetite-fayalite. The bonding behaviour of the chromium atoms in chromous silicate (and the reason why this compound does not form an olivine structure as expected) could only be elucidated through parallel study of isotypic cadmium silicate. And an understanding of the transition behaviour of garnets, a major mantle mineral group, can be developed through studying the behaviour of transition-metal analogues.

Further the concept of analogous behaviour extends even further than the simple concept of analogue structures. Thus the phase transition in calcium silicate has provided us with a first opportunity to study in the crystalline state the change in coordination of silicon by oxygen, a process that probably dominates the physical properties of silicate melts deep in the upper mantle of the Earth. In contrast, the study of densified germanium oxide glasses provides insights into the driving forces for melting and amorphisation of silicate minerals.

It is clear from the examples presented here that the way forward for crystal chemical studies for application to Earth Sciences is, in actual fact, away from mineralogy and Earth materials. Studies must be pursued over a much wider field of structures and chemistries, even those of no obvious direct application to mineralogy, because physical behaviour knows no such artificial boundaries as are frequently drawn between mineralogy, physics and chemistry.