Résumé
Slates are low-grade metamorphic argillaceous rocks which have developed an intense tectonic fabric, referred to as slaty cleavage. Due to their unique properties they have long been used as building materials, especially tiles for roofs and floors. The fabric, characterized by strong alignment of platy phyllosilicate crystals, produces extreme anisotropy that has been of great interest in structural geology: What different fabric types are observed? How do they form? Here we review crystal preferred orientation (CPO) and add new information to establish a comprehensive base for future studies. Slates are composed of fine-grained phyllosilicates (dominantly white mica and chlorite, sometimes chloritoid), quartz and accessories. Texture analyses of these polyphase rocks with high energy synchrotron X-ray diffraction and high resolution EBSD reveal strong alignment of mica and chlorite, in some cases exceeding any previously reported texture strength, including metals. Interestingly, the CPO of quartz, with highly flattened grains (shape preferred orientation or SPO), is practically random. Recrystallization microstructures observed by scanning electron microscopy suggest that the phyllosilicate preferred orientation is largely caused by crystal growth under stress, and that quartz was subject to pressure solution and reprecipitation, producing an anisotropic shape. While phyllosilicates preferred orientation in slates is much higher than reports for shale, schists and gneiss, elastic seismic anisotropy for slates is similar to shales where a significant component is due to aligned porosity.