High-Resolution Electron Microscopy of Olivine-Magnetite Interfaces
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HIGH-RESOLUTION ELECTRON MICROSCOPY OF OLIVINE-MAGNETITE INTERFACES STUART MCKERNAN, C. BARRY CARTER, DANIEL RICOULT*, AND A. G. CULLIS** Department of Materials Science and Engineering, Bard Hall, Cornell University, Ithaca, NY 14853, *Coming-Europe, 7bis Avenue de Valvins, Avon F77210, France, **RSRE St. Andrews Rd. Worcs., WR14 3PS, England. ABSTRACT The oxidation of iron-rich olivine to produce magnetite is a model system for the study of phase transitions involving mass transport. High-resolution lattice images of have been obtained from magnetite precipitates in naturally modified iron-rich olivines. The magnetite/olivine interface is shown to be extremely sharp. Steps and misfit dislocations are present at the interface. INTRODUCTION The oxidation of iron-rich olivine at elevated temperatures to produce magnetite is a model system for the study of phase transitions involving mass transport. The two structures (olivine and spinel) exist in orientations such that the planar spacing for several low index planes in each phase is very closely matched. The oxygen sublattice of the two systems may be continuous across the phase boundary, although some slight displacement of the atoms occurs in the transformation from one phase to the other. The cation sublattice of both structures contains structural vacancies which facilitates the movement of cations to the transformation front. The transformation from olivine to magnetite requires the transport of Fe ions to the spinel phase, and the transport of Mg and Si ions away from the growing magnetite precipitates. A knowledge of the interface structure and the structure of defects present at that interface, such as dislocations, steps and ledges, is essential in determining the mechanisms for the phase transformation. The morphology of second phase particles in the olivine matrix depends on several factors; the initial iron concentration in the olivine, the time and temperature of the heat treatment and the oxygen partial pressure. An understanding of the precipitate structure may help to deconvolute these different effects, and thus enable the thermal history of naturally occurring olivines to be determined. High-resolution lattice images have been obtained using a JEOL 4000EX electron microscope from samples of natural iron-bearing olivine from San Carlos. This olivine has been heat-treated at 10000 C for 1 hour to internally oxidize the material. Transmission electron microscope images from this material show several different precipitates which heavily decorate the dislocations present in the material. Hematite, magnetite, tridymite, and enstatite are among the minerals which have been identified in he decorated regions [1,3]. In addition to these precipitates nucleated at dislocations, small homogeneously nucleated needle-shaped magnetite precipitates are also observed when the electron beam is oriented parallel to the olivine [010] direction. Similar phase distributions and morphologies have been observed in olivine specimens heat-treated in the laboratory [e.g. 1]
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