- © The Mineralogical Society Of America
Minerals are often riddled with microstructures when observed under the electron microscope. Although the observation and classification of microstructures such as twin boundaries, anti-phase boundaries, exsolution lamellae etc., has been a longstanding activity of mineralogists and crystallographers it has only been very recently that we started to understand the enormous importance of microstructures for the physical and chemical behaviour of minerals.
Let us take a twin boundary as an example. Illustrations of twinned minerals have decorated books on mineralogy for more than a century. A first major step forward was made when Burger (1945) noticed that twinning can have different origins, namely that it may be a growth phenomenon, created mechanically or be the outcome of a structural phase transition. Much research was then focussed on the latter type of twins, which Burger called ‘transformation twins.’ Transformation twin patterns can be reproduced rather well in a larger number of technically important materials, such as martensitic steels, and it was understood very quickly that they dominate much of the mechanical behaviour. Ferroelectric and ferromagnetic domain patterns are equally important for electronic memory devices while ferroelastic, hierarchical domain structures have fascinated mathematicians and physicists because of the apparent simplicity and universality of their elastic interactions.
A novel aspect relates to the chemical properties of such ‘mesoscopic’ structures. The term ‘mesoscopic’ stands for ‘in-between scales,’ as bracketed by atomistic and macroscopic length scales. The two key observations are the following. Firstly, it was shown experimentally that twin boundaries can act as fast diffusion paths. Aird and Salje (1998, 2000) succeeded in showing enhanced oxygen and sodium diffusion along twin walls in the perovskite like structure NaxWO3−δ. They also showed that the electronic structure of the twin boundary is different from that of the bulk, results echoed by structural …