Systematics of the post-spinel tranisition in Fe-bearing compositions

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Phases with the spinel structure occur in many fundamental chemical systems of geological importance and a number of these are known to undergo one or more phase transformations at transition zone and lower mantle pressures. The transformation mechanisms and crystal chemical factors controlling the formation of particular postspinel structures are poorly understood, as are the P-T stabilities of the post-spinel phases. Our goal is to address this knowledge deficit through a study combining in situ experiments (DAC and multi-anvil) at high P and T (10-25 GPa and up to 1600°C), with TEM characterisation (microstructures) and atomistic computer simulations. We will concentrate on relevant Fe-bearing systems: Fe3O4 (magnetite), Fe3O4-FeCr2O4 (magnetite-chromite) and FeAl2O4 (hercynite). Different theoretical approaches will be tested by comparison with experimental data. We seek to develop thermodynamic descriptions of the transformations, as well as understanding their mechanisms (by TEM). The resulting thermodynamic data will be used to calculate phase relations in these fundamental systems at high P and T. The implications of these transformations for natural systems will be explored.