Rheology of Earth Materials :

Closing the gap between timescales in the laboratory and in the mantle

22 July 2020: Ferroelasticity in the mantle: mobility of twin walls in bridgmanite

Bridgmanite (Mg,Fe)(Si,Al)O3 is the most abundant mineral in the Earth’s lower mantle, where it crystallizes in an orthorhombic perovskite phase. This peculiar structure makes it a ferroelastic material, capable of reversible deformation thanks to the motion of {110} twin walls. These walls can interact with seismic waves, and be responsible for seismic attenuation.

We modelled (110) twin walls in bridgmanite of composition MgSiO3, and studied their properties at the atomic scale. A twin wall forms when two crystals share the same [001] axis and have inverted [100] and [010] directions, as shown in the figure below. It is essentially characterized by a change in the tilt of SiO6 octahedra, the atomic environments remaining similar to those in the bulk. Atomic planes of the two crystals meet with an angle about 182.66°, and the width of the twin boundary was estimated to 20 Å.

Twin walls move under an applied load by small changes in the tilt of SiO6 octahedra. We applied cyclic loading to our simulation sample, and monitored the stress and strain (see graph below). Twin walls move reversibly, but do not return to their original position when the stress is relieved, thus causing permanent deformation aka ferroelastic behaviour. Detailed analysis of the reaction path for twin wall motion revealed an activation energy about 20 meV, very small compared to the temperatures typical of the Earth’s lower mantle.

Atomic-scale structure of a twin wall in bridgmanite. Mg ions are represented as orange spheres, and SiO6 octahedra in blue. The graph on the right-hand side shows the ferroelastic behaviour of the simulation sample when submitted to cyclic loading

These results are now published in the following article:

P. Hirel, P. Carrez & P. Cordier (2020) Ferroelasticity in MgSiO3 bridgmanite: assessing the structure and mobility of (-110) twin walls with atomic-scale simulations. Scripta Materialia, 188, 102-106. https://doi.org/10.1016/j.scriptamat.2020.07.016