Microstructures and Anisotropy of Slates from Northern China

Slates transform from shales at relatively low-grade metamorphic conditions. They often reveal highly anisotropic microstructures and very strong crystal alignment that must be considered in seismic modeling and engineering construction. In this paper, we investigate nine slate samples from four regions in northern China: Fangshan, Beijing; Xushui, Hebei; Damao Qi, Inner Mongolia; and Zhengxiangbai Qi, Inner Mongolia. The microstructural characteristics were analyzed with scanning electron microscopy and explored with digital crystal size distribution analysis. Preferred crystal orientation characteristics of slate minerals were investigated with high-energy synchrotron X-ray diffraction and subsequent Rietveld refinement. This research shows that the main components of slates in this study are quartz, muscovite, chlorite, and minor orthoclase. In terms of morphology, muscovite, chlorite, and quartz are strongly elongated and oriented. The crystallographic orientation of sheet silicates is very strong, exceeding 100 multiples of random distribution for chlorite from Fangshan. However, quartz with a preferred strong shape orientation has a crystallographic preferred orientation close to random. The preferred orientation characteristics of minerals serve as a basis for calculating elastic properties and anisotropies of the Chinese slate samples that contribute significantly to seismic anisotropy documented in northern China.