Ordering state in orthopyroxene as determined by precession electron diffraction

Abstract

Fe 2+ 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 mineral. It is generally obtained by single-crystal X-ray diffraction, which is limited in spatial resolution. In this work, we determine the cationic distribution at a submicrometer scale in a transmission electron microscope using precession electron diffraction. Two orthopyroxene samples coming from the same metamorphic rock are studied, a naturally ordered one and a disordered one. The latter was obtained from the ordered sample by annealing at high temperature and rapid quenching. Both samples have been first studied in X-ray diffraction and then in precession electron diffraction. Intensities recorded in zone-axis precession electron diffraction experiments have been quantitatively analyzed and compared to simulations, taking into account dynamical interactions between diffracted beams. Our structure refinement results are in good agreement with those obtained by single-crystal X-ray diffraction. They enable to distinguish between the ordered sample and the disordered one in terms of the observed molar fractions of Fe at M1 and M2 sites. We discuss the sensitivity of the method as a function of experimental parameters. The larger dispersion of the results obtained on the ordered specimen is attributed to structural heterogeneities inherent to the sample.

Precession electron diffraction – a topical review

“…With precession, the ratio changed monotonically with thickness (unlike the case without precession) and thus was shown to be well suited for order parameter measurements at the nanoscale. In a subsequent study, the ordering state in orthopyroxene (Jacob et al, 2013) was investigated. Here, ZA-PED patterns were used to determine the Fe and Mg distributions on different octahedral sites in the orthopyroxene structure.…”

Synthesis of a quenchable high-pressure form of magnetite (h-Fe3O4) with composition Fe1(Fe2+0.75Mg0.26)Fe2(Fe3+0.70Cr0.15Al0.11Si0.04)2O4

“…Structure residuals and GooF are high when compared with X-ray diffraction, but typical for electron diffraction data . In the present case dynamical effects and structure residuals are emphasized by the high density of the material and the thickness of the samples -estimated about 200-400 nm for µ-crystals and 150 nm for the FIB cuts Jacob et al 2013). Table 5 shows the observed d-spacings and corresponding angles between vectors for both samples.…”

Solving difficult structures with electron diffraction

“…1). Using PED intensities, crystal structures can be solved by a combination of phasing and structure refinement, where the R factor can be reduced to less than 10% by further including dynamic effects (Palatinus et al, 2013;Jacob et al, 2013).…”