Quantitative analysis of precession electron diffraction: application to the determination of ordering state in pyroxene.

• Main Authors: Cuvillier P, Jacob D, Palatinus L, Leroux H, Domeneghetti MC, CAMARA ARTIGAS, Fernando
• Other Authors: Cámara F
• Format: Conference Object
• Language: English
• Published: Royal Microscopical Society 2012 2012
• Subjects: Precession electron diffraction; orthopyroxene; X-ray diffraction; Antarc*; Antarctica
• Online Access: http://hdl.handle.net/2318/131577; http://www.emc2012.org.uk/documents/Abstracts/Abstracts/EMC2012_0095.pdf

Precession electron diffraction (PED) has recently renewed the interest in electron diffraction for structural and microstructural analysis [1]. The main advantage is that, being sequentially acquired out of a zone-axis orientation, diffracted intensities are less affected by dynamical interactions. This has already been proved to be useful for microstructures characterization in cases where crystal exhibits low symmetry departure [2]. For quantitative analysis of intensities and comparison with calculated values, dynamical interactions have to be considered [3]. In this work, we address the sensitivity of PED intensities to determine cation ordering in complex mineral structures such as orthopyroxene [OPX; (Mgx Fe2-x)Si2 O6] from igneous origin. In this structure, the Mg and Fe2+ ordering process among two non-equivalent crystallographic sites is related to the kinetics of diffusion process, making the mineral a potential geo-thermometer for deciphering the formation conditions of terrestrial or extra-terrestrial rocks. The gain in spatial resolution associated with the use of electron beam instead of X-rays for diffraction experiments open access to mineral samples of limited size of with complicated microtextures. The two studied samples are monocrystals of natural OPX from granulite rocks of the Wilson Terrane in Antarctica [4]. Composition as measured by microprobe analysis is close to Mg1.4Fe0.6Si2 O6, giving a Mg/(Mg+Fe) ratio close to 0.7. One crystal has been kept untreated (plain natural), and shows an ordered structure. The other one has been heated for 48h at 1000°C and then quenched, thus obtaining a disordered structure. The degree of order of both single crystal grains have been characterized by XRD structure refinement. TEM samples have been extracted using Focused Ion Beam from mono-crystalline grains previously studied by XRD. The space group is orthorhombic Pbca with a=1.8337, b=0.8971 and c=0.5232 nm for the untreated crystal and a=1.8291, b=0.888 and c=0.5207 nm for the heat-treated one. ...