Reliability of multigrain indexing for orthorhombic polycrystals above 1 Mbar: application to MgSiO₃ post-perovskite

Abstract

This paper describes a methodology for characterizing the orientation and position of grains of an orthorhombic polycrystalline material at high pressure in a diamond anvil cell. The applicability and resolution of the method are validated by simulations and tested on an experimental data set collected on MgSiO 3 post-perovskite at 135 GPa. In the simulations, $95% of the grains can be indexed successfully with $80% of the peaks assigned. The best theoretical average resolutions in grain orientation and position are 0.02 and 1.4 mm, respectively. The indexing of experimental data leads to 159 grains of postperovskite with 30% of the diffraction peaks assigned with a 0.2-0.4 resolution in grain orientation. The resolution in grain location is not sufficient for in situ analysis of spatial relationships at high pressure. The grain orientations are well resolved and sufficient for following processes such as plastic deformation or phase transformation. The paper also explores the effect of the indexing parameters and of experimental constraints such as rotation range and step on the validity of the results, setting a basis for optimized experiments. research papers J. Appl. Cryst. (2017). 50, 120-130 Christopher Langrand et al. Reliability of multigrain indexing 121

Exploring microstructures in lower mantle mineral assemblages with synchrotron x-rays

“…Generally speaking, the orientations and magnitudes of deviatoric strain are not overly sensitive to factors like the quality of detector calibration and the instrument’s stability. The measured centroidal positions and hydrostatic strains, however, are sensitive and thus more challenging to determine accurately ( 36 ). The size of the rotation range increases accuracy globally, as it leads to both more independent Bragg reflections and more variation in projection directions; this limitation is made apparent in the case of fp in this study, where after extended heating, hundreds of unique orientations were identified, but only a limited number could be constrained spatially.…”

Evolution of grain sizes and orientations during phase transitions in hydrous Mg2SiO4

“…Recently, multigrain crystallography (MGC) was developed for in situ diamond anvil cell (DAC) experiments [Nisr et al, 2012;Zhang et al, 2013;Nisr et al, 2014;Langrand et al, 2016]. The method allows in situ measurements of crystallographic structures, grain orientations, and grain statistics at high pressure during dynamic processes such as deformation or phase transformation [Rosa et al, 2015].…”

Textures Induced by the Coesite‐Stishovite Transition and Implications for the Visibility of the X‐Discontinuity

“…Finally, we use MTEX Mainprice, Bachmann, Hielscher, Schaeben, & Lloyd, 2015) in order to fit an orientation distribution function (ODF) to the list of single-grain orientations and plot the results as pole figures. Estimated errors in the grain orientations are typically below 0.4° in simulated datasets (Langrand et al, 2017) and they are not expected to exceed 1-2° for real data here.…”

“…We provide, here, a short description of the MGC data analysis procedure. Details can be found in Langrand et al (2017). We used the open-source software package FABLE-3DXRD (https://github.com/FABLE-3DXRD) complemented with our custom scripts from the TIMEleSS-MGC-tools (https://github.com/FABLE-3DXRD/ TIMEleSS).…”

“…In this study, we address this issue with an in‐situ study of LPO in silica across the coesite‐stishovite transformation at mantle conditions by applying a novel X‐ray diffraction technique based on multigrain crystallography (MGC) analysis in a laser‐heated diamond‐anvil cell (LH‐DAC). The MGC technique, also known as 3‐dimensional X‐ray diffraction (3D‐XRD), provides information on the microstructural properties of the sample, including the grain size and grain orientation of individual grains in a polycrystalline aggregate (Langrand et al., 2017; Sørensen et al., 2012). We apply the results to evaluate the effect of textures on the impedance contrast across the coesite‐stishovite transition and further identify the seismic signature of the transition.…”