Anisotropy of quartz mica schist based on quantitative extraction of fabric information

Abstract

In this study, comprehensive laboratory experiments, such as thin-section and optical section analyses and P wave velocity and uniaxial compression tests, are performed on three types of quartz mica schist samples. The thin-section analysis reveals that the schist predominately consists of flaky mica and granular minerals, including quartz, feldspar, and calcite. The two types of minerals, both presenting a certain degree of orientation, are arranged to form a quasi-bedded structure as a whole, which essentially leads to macroscopic physicomechanic anisotropy. By means of optical section observation, directional microfissures are found to mainly be distributed alongside the edge of the oriented mica layer, while defects in other parts exhibit irregular orientation. Moreover, quantitative information is extracted from gray images by Image-Pro Plus (IPP), and then, relevant indicators are proposed to quantitatively analyze the differences in the content, morphology, and distribution characteristics of minerals and defects among the three types of samples. P wave velocity measurements of dry cylindrical specimens with different schistosity angles α show that the minimum value lies in the direction of the transverse schistosity planes (α = 90°) and that the P wave velocity increases with the decrease of the schistosity angle. There is an obvious linear positive correlation between the orientation coefficient of mica, which is adopted to indirectly reflect the orientation degree of microfissure, and the 0.5 power of the velocity anisotropy coefficient. The uniaxial compression test of specimens reveals the anisotropic characteristics of the peak strength, failure mode, and crack initiation and propagation for the schist samples. In particular, with the change of the schistosity angle, the peak strength and crack initiation strength have a U shape, characterized by a minimum value at α = 30° and maximum value at α = 0° or 90°. Directional microfissures and irregular defects with a dominant angle control the crack initiation of the specimens with 0° ≤ α ≤ 45° and 45° < α ≤ 90°, respectively. The weak edge of the directional mica layers has a more or less guiding effect on crack propagation when compressive loading is applied obliquely to the schistosity planes, while crack propagation is independent of weak edges when perpendicular to the planes. Three failure modes occur in the specimens with the change of the schistosity angle: tension-splitting failure, shear-slip failure, and shear failure. It is found that the mechanical anisotropy properties of the schist samples are closely associated with the content, morphology, and distribution characteristics of minerals and defects based on the analyses of the macro- and microquantitative data.