| Summary: | Transmission electron microscopy (TEM) has been used to investigate the assumptions made in several branches of geophysics, concerning the behaviour of materials. TEM observations made on shocked olivines and pyroxenes show that a number of the assumptions made in the application of the results of experimental studies of shock-induced phase transformations are incorrect. TEM studies of titanomagnetites have also shown that the interpretation of the magnetic properties of these oxides may be complicated by their exsolution microstructures. TEM of the black veins of the Tenham meteorite, reveals that the minerals have undergone varying degrees of shock. Orthopyroxene has been transformed to its high-density, garnet-structure polymorph, majorite, and olivine has been transformed to its high-density, spinel polymorph. The majorite, which occurs with both equant and dendritic habit, is associated with a glassy phase, from which it crystallized, and with a microcrystalline clinopyroxene aggregate, produced from the majorite upon the release of pressure. Similarly the spinel polymorph of olivine, has subsequently inverted to the B-phase polymorph, on pressure release. The mechanisms for high-pressure, high-strain rate transformations, and their implications to shock experiments, are discussed, and it is concluded that many of the inferences made from the results of shock experiments are likely to be in error. A survey of the exsolution microstructures, developed in titanomagnetites, is presented. These microstructures generally consist of a three-dimensional, lamellar framework of ulvospinel and magnetite. The implications of exsolution and oxidation, to the magnetic properties of titanomagnetites, are discussed . The likely mechanisms of microstructural development in titanomagnetites are investigated, and a kinetic model to describe the evolution of exsolution-derived microstructures is advanced. In order that the kinetic model could be evaluated, the diffusion and phase data for the titanomagnetite system was investigated, and finally the sizes of the ulvospinel lamellae, developed in some titanomagnetites were used to determine the thermal history of their host rocks, by solving the diffusion equation describing lamellar growth.
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