Fe2+ and Mg distribution on octahedral M1 and M2 sites of the orthopyroxene structure is an indicator of the cooling rate and closure temperature of the studied mineral [1]. These data are of great importance to retrieve the formation conditions, whatever the terrestrial or extra-terrestrial origin. The cationic distribution is generally obtained by X-ray diffraction (XRD) followed by structure refinement, leading to the atomic positions and sites occupancy with a good accuracy. Nevertheless, XRD is limited in spatial resolution. Contradictory results in cooling rate determination based on sites occupancy as determined by XRD have been explained by the occurrence of microstructural features such as local composition variations, exsolution lamellae and Guinier-Preston zones [2,3,4]. These features can only be revealed by imaging in a transmission electron microscope (TEM). XRD analysis then leads to averaged information, which may induce misinterpretation. In this work, we present results on site occupancy determination obtained at a microscopic scale in a TEM using precession electron diffraction. Studied samples, as study cases, are monocrystals of terrestrial metamorphic orthopyroxenes. Our analysis allows to distinguish unambiguously an ordered sample (natural, non-treated) from a disordered one (heat-treated).
Fe-Mg ordering in orthopyroxene studied at a microscopic scale using precesion electron diffraction
CAMARA ARTIGAS, Fernando
2012-01-01
Abstract
Fe2+ and Mg distribution on octahedral M1 and M2 sites of the orthopyroxene structure is an indicator of the cooling rate and closure temperature of the studied mineral [1]. These data are of great importance to retrieve the formation conditions, whatever the terrestrial or extra-terrestrial origin. The cationic distribution is generally obtained by X-ray diffraction (XRD) followed by structure refinement, leading to the atomic positions and sites occupancy with a good accuracy. Nevertheless, XRD is limited in spatial resolution. Contradictory results in cooling rate determination based on sites occupancy as determined by XRD have been explained by the occurrence of microstructural features such as local composition variations, exsolution lamellae and Guinier-Preston zones [2,3,4]. These features can only be revealed by imaging in a transmission electron microscope (TEM). XRD analysis then leads to averaged information, which may induce misinterpretation. In this work, we present results on site occupancy determination obtained at a microscopic scale in a TEM using precession electron diffraction. Studied samples, as study cases, are monocrystals of terrestrial metamorphic orthopyroxenes. Our analysis allows to distinguish unambiguously an ordered sample (natural, non-treated) from a disordered one (heat-treated).
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