Anisotropy of Earth's D″ layer and stacking faults in the MgSiO3 post-perovskite phase

Abstract

The post-perovskite phase of (Mg,Fe)SiO3 is believed to be the main mineral phase of the Earth's lowermost mantle (the D″ layer). Its properties explain1,2,3,4,5,6 numerous geophysical observations associated with this layer—for example, the D″ discontinuity7, its topography8 and seismic anisotropy within the layer9. Here we use a novel simulation technique, first-principles metadynamics, to identify a family of low-energy polytypic stacking-fault structures intermediate between the perovskite and post-perovskite phases. Metadynamics trajectories identify plane sliding involving the formation of stacking faults as the most favourable pathway for the phase transition, and as a likely mechanism for plastic deformation of perovskite and post-perovskite. In particular, the predicted slip planes are {010} for perovskite (consistent with experiment10,11) and {110} for post-perovskite (in contrast to the previously expected {010} slip planes1,2,3,4). Dominant slip planes define the lattice preferred orientation and elastic anisotropy of the texture. The {110} slip planes in post-perovskite require a much smaller degree of lattice preferred orientation to explain geophysical observations of shear-wave anisotropy in the D″ layer.

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Figure 1: MgSiO 3 polytypes found by metadynamics.
Figure 2: Enthalpies (relative to Pv, per formula unit) of MgSiO 3 polytypes as a function of pressure.
Figure 3: Activation barrier for the Pv–pPv transition at 120 GPa.

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Acknowledgements

Calculations were performed at ETH Zurich and CSCS (Manno). A.R.O. is grateful to P. Cordier, T. Ungar, G. Ferraris, T. Balic-Zunic, E. Makovicky and C. Thomas for discussions on various aspects of this work. Author Contributions A.R.O. designed and performed this work and wrote the paper. Many ideas on plasticity and phase transformation mechanisms arose from discussions between A.R.O., R.M., A.L. and M.P.; R.M. and P.R. assisted A.R.O. in technical aspects of this work.

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Correspondence to Artem R. Oganov.

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Supplementary information

Supplementary Figure 1

This figure shows ab- and bc- projections of perfect post-perovskite (top) and post-perovskite with the {010} stacking fault (bottom). This stacking fault turns out to be very unfavourable. As discussed in the paper, the {110} stacking faults are preferred instead. SiO6 octahedra are shown in blue, Mg atoms are large pink spheres, Si atoms are small blue spheres, O atoms are small red spheres. (PDF 131 kb)

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Oganov, A., Martoňák, R., Laio, A. et al. Anisotropy of Earth's D″ layer and stacking faults in the MgSiO3 post-perovskite phase. Nature 438, 1142–1144 (2005). https://doi.org/10.1038/nature04439

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