APPLYING ADVANCED MINERALOGICAL METHODS TO UNDERSTAND ANISOTROPY IN ROCKS (Invited Presentation)

Crystals are anisotropic – they are different when viewed in different directions – due to their periodic lattice structure. When crystals are aligned in rocks then also rocks display anisotropic properties such as propagation of seismic waves in the Earth. The focus of this presentation is to show how advanced methods used in mineralogy can help us quantify the alignment of minerals, like measurements with synchrotron X-ray diffraction, neutron scattering and electron microscopy. An example is illite and smectite in shales where deformation occurred mainly by compaction. By measuring preferred orientation one can predict anisotropic elastic properties of the macroscopic aggregate that are highly relevant, e.g. for geophysical prospecting. A unique discovery has been the extreme preferred orientation in slates, fine-grained metamorphic rocks composed largely of muscovite, chlorite and quartz. The alignment of muscovite exceeds that of any other material, including metal foils and epitaxial films. A more remote example is the core-mantle boundary where seismologists have documented strong anisotropy which can likely be attributed to alignment of post-perovskite crystals (MgSiO₃) during slab subduction. This requires delicate investigations to determine deformation mechanisms at extreme pressure and temperature which can be simulated in a diamond anvil cell. With knowledge about deformation mechanisms, polycrystal plasticity theory can be used to model the seismically observed anisotropy in the deep Earth.