Texture, plastic deformation, and phase transformation mechanisms in perovskite and post-perovskite are of general interest for our understanding of the Earth's mantle. Here, the perovskite analogue NaCoF3 is deformed in a resistive-heated diamond anvil cell (DAC) up to 30 GPa and 1013 K. The in situ state of the sample, including crystal structure, stress, and texture, is monitored using X-ray diffraction. A phase transformation from a perovskite to a post-perovskite structure is observed between 20.1 and 26.1 GPa. Normalized stress drops by a factor of 3 during transformation as a result of transient weakening during the transformation. The perovskite phase initially develops a texture with a maximum at 100 and a strong 010 minimum in the inverse pole figure of the compression direction. Additionally, a secondary weaker 001 maximum is observed later during compression. Texture simulations indicate that the initial deformation of perovskite requires slip along (100) planes with significant contributions of {110} twins. Following the phase transition to post-perovskite, we observe a 010 maximum, which later evolves with compression. The transformation follows orientation relationships previously suggested where the c axis is preserved between phases and hh0 vectors in reciprocal space of post-perovskite are parallel to [010] in perovskite, which indicates a martensitic-like transition mechanism. A comparison between past experiments on bridgmanite and current results indicates that NaCoF3 is a good analogue to understand the development of microstructures within the Earth's mantle.

Bridgmanite, the dominant mineral in the Earth's lower mantle, crystallizes in the perovskite structure and transforms into post-perovskite at conditions relevant for the D '' layer. This transformation affects the dynamics of the Earth's lowermost mantle and can explain a range of seismic observations. The thickness over which the two phases coexist, however, can extend over 100 km, casting doubt on the assignment of the observed seismic boundaries. Here, experiments show that the bridgmanite to post-perovskite transition in (Mg-0.86,Fe-0.14) SiO3 is fast on geological timescales. The transformation kinetics, however, affects reflection coefficients of P and S waves by more than one order of magnitude. Thick layers of coexisting bridgmanite and post-perovskite can hence be detected using seismic reflections. Morever, the detection and wave period dependence of D '' reflections can be used to constrain significant features of the Earth's lowermost mantle, such as the thickness of the coexistence layer, and obtain information on temperature and grain sizes.

At P/T conditions of the D” layer, bridgmanite transforms into its high-pressure phase of (Mg,Fe)SiO3 postperovskite (pPv). Observations of seismic anisotropy in D” are inferred to arise from textures and microstructures within pPv. Specifically, mantle flow is though to cause pPv to deform, creating lattice-preferred orientations. However, debates emerged in the literature whether experimentally observed textures were induced by plastic deformation of the sample or by phase transformation from a previous phase and whether this could explain the observed patterns of anisotropy in the lowermost mantle.

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 µm, respectively. The indexing of experimental data leads to 159 grains of post-perovskite 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.