Crystallographic and shape preferred orientation producing anisotropy in slates from Northern Spain

Slates are common metasedimentary rocks in the Iberian Peninsula, ranging in deposition age from Proterozoic to Ordovician. Spain is currently the primary producer of high-quality slate products. The excellent cleavage is related to microstructures with very strong preferred orientation of white mica and chlorite. In this study we explore preferred orientation of eleven slates and one phyllite of different composition and metamorphic grade from Northern Spain with high energy synchrotron X-ray diffraction and study microstructures with scanning electron microscopy. While phyllosilicates display high crystallographic and shape preferred orientation, quartz, with also highly flattened grains, has in most samples a random crystallographic orientation. The microstructural and texture data are used to model anisotropic seismic properties with a self-consistent method and to explore the influence of crystal shapes on elastic anisotropy. Anisotropy for these slates is very high compared with gneiss and shale and, due to its prevalence, slate anisotropy contributes significantly to seismic anisotropy in the upper crust.