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All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Deformation and Transformation Textures in the NaMgF3 Neighborite—Post-Perovskite System

The D″ region of the lower mantle, which lies just above the core–mantle boundary, is distinct from the bulk of the lower mantle in that it exhibits complex seismic heterogeneity and seismic anisotropy. Seismic anisotropy in this region is likely to be largely due to the deformation-induced texture (crystallographic preferred orientation) development of the constituent mineral phases. Thus, seismic anisotropy can provide a marker for deformation processes occurring in this dynamic region of the Earth. Post-perovskite-structured (Mg,Fe)SiO3 is believed to be the dominant mineral phase in many regions of the D”. As such, understanding deformation mechanisms and texture development in post-perovskite is important for the interpretation of observed seismic anisotropy. Here, we report on high-pressure diamond anvil cell deformation experiments on NaMgF3 neighborite (perovskite structure) and post-perovskite. During deformation, neighborite develops a 100 texture, as has been previously observed, both in NaMgF3 and MgSiO3 perovskite. Upon transformation to the post-perovskite phase, an initial texture of {130} at high angles to compression is observed, indicating that the {100} planes of perovskite become the ~{130} planes of post-perovskite. Further compression results in the development of a shoulder towards (001) in the inverse pole figure. Plasticity modeling using the elasto-viscoplastic self-consistent code shows this texture evolution to be most consistent with deformation on (001)[100] with some contribution of glide on (100)[010] and (001)<110> in NaMgF3 post-perovskite. The transformation and deformation mechanisms observed in this study in the NaMgF3 system are consistent with the behavior generally observed in other perovskite–post-perovskite systems, including the MgSiO3 system. This shows that NaMgF3 is a good analog for the mantle bridgmanite and MgSiO3 post-perovskite.

Geochemistry & Geophysics↗
DOE OSTI2020

Materials Data on NaMgF3 by Materials Project

NaMgF3 crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. Na1+ is bonded in a 8-coordinate geometry to eight F1- atoms. There are a spread of Na–F bond distances ranging from 2.31–2.58 Å. Mg2+ is bonded to six F1- atoms to form a mixture of corner and edge-sharing MgF6 octahedra. The corner-sharing octahedral tilt angles are 42°. There are two shorter (2.00 Å) and four longer (2.03 Å) Mg–F bond lengths. There are two inequivalent F1- sites. In the first F1- site, F1- is bonded in a 5-coordinate geometry to three equivalent Na1+ and two equivalent Mg2+ atoms. In the second F1- site, F1- is bonded to two equivalent Na1+ and two equivalent Mg2+ atoms to form distorted corner-sharing FNa2Mg2 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on NaMgF3 by Materials Project

NaMgF3 is Orthorhombic Perovskite structured and crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. Na1+ is bonded in a 8-coordinate geometry to eight F1- atoms. There are a spread of Na–F bond distances ranging from 2.32–2.74 Å. Mg2+ is bonded to six F1- atoms to form corner-sharing MgF6 octahedra. The corner-sharing octahedral tilt angles are 30°. All Mg–F bond lengths are 2.01 Å. There are two inequivalent F1- sites. In the first F1- site, F1- is bonded in a 5-coordinate geometry to three equivalent Na1+ and two equivalent Mg2+ atoms. In the second F1- site, F1- is bonded to two equivalent Na1+ and two equivalent Mg2+ atoms to form distorted corner-sharing FNa2Mg2 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on NaMgF3 by Materials Project

NaMgF3 is (Cubic) Perovskite structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Na1+ is bonded to twelve equivalent F1- atoms to form NaF12 cuboctahedra that share corners with twelve equivalent NaF12 cuboctahedra, faces with six equivalent NaF12 cuboctahedra, and faces with eight equivalent MgF6 octahedra. All Na–F bond lengths are 2.79 Å. Mg2+ is bonded to six equivalent F1- atoms to form MgF6 octahedra that share corners with six equivalent MgF6 octahedra and faces with eight equivalent NaF12 cuboctahedra. The corner-sharing octahedral tilt angles are 0°. All Mg–F bond lengths are 1.98 Å. F1- is bonded in a distorted linear geometry to four equivalent Na1+ and two equivalent Mg2+ atoms.

36 MATERIALS SCIENCE↗