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Structure-property relations of minerals

S.A.T. Redfern

My work is focussed on developing and testing models that relate the properties of solids across many length scales. It ranges from investigating the influence of microstructure in minerals (at a scale of millionths of a metre) on the anelastic properties of the deep mantle (at a scale of millions of metres), through understanding crystal chemical controls on the atomic structure and stability of minerals and biominerals, to exploring routes to determine chemical activities in solid solutions.

This may be considered under the following themes:

a) Structural physics and chemistry of mesoscopic materials

How do domain walls generated below symmetry-breaking phase transitions influence the elastic properties of minerals? Using novel apparatus designed and constructed in house we are able to follow the anelastic response of minerals below phase transitions, using experimentally observed critical behaviour in elastic loss and moduli to develop new models for the real behaviour of microstructured minerals in the Earth. This has led to the first observations of superelastic behaviour in mantle-relevant perovskites at seismic frequencies. A related theme is the investigation of chemical transport along domain walls by direct experimental observation, measuring Li diffusion in microstructured quartz (Figure 1).

b) Structural controls on stabilities at high P/T

What are the processes that control the phase stabilities of minerals under the conditions of the geotherm? In many cases these depend upon the behaviour of hydrogen, and changes in hydrogen bonding. In a larger collaborative programme I am involved in the development of new apparatus to allow neutron diffraction of minerals at high-P/T in situ, providing a way forward for the direct observation of the nature of hydrogen in solids at the conditions of the upper mantle.

c) Novel routes to new materials

Nature is able to provide several clues as to the synthesis and exploration of technologically useful materials. Biominerals display physical properties far in excess of the constituent parts, with control of structure exercised principally during growth at the surface. We have begun an exploration of the nature of the surface control of carbonates in the formation of biominerals, with the potential to apply these controls in the synthesis of new structures. On the other hand, the potential of high-P/T synthesis via metastable precursors provides a route for the development of novel refractory oxides relevant to the problems of chemical and nuclear waste disposal, as well as structures of interest in materials chemistry more widely.

Figure 1: The behaviour of the elastic storage modulus and elastic loss modulus of a strontium-calcium titanate perovskite as a function of temperature through the cubic-tetragonal phase transition. The large elastic loss ("tan delta") arises from the movement of domain walls under applied stress in the three-point bend geometry of the experiment, and their interaction with pinning centres and grain boundaries. The dynamics of domain movement and relaxation behave according to a Debye model, with a peak in tan delta below which domain wall movement freezes.