Biotite effect on the anisotropic properties of gneiss in Eastern Himalayas, China

Intact rocks with layered microstructures, such as gneiss, exhibit strong anisotropy. Although anisotropy in the macroscopic strength of gneiss has been widely reported, the role of microstructures in tensile mechanics remains largely unclear. Utilizing a range of methodologies, it was determined that the tensile strength, deformation, and fracturing behavior of Himalayan gneiss are predominantly influenced by biotite. In contrast to the behavior of other transversely isotropic rocks, the apparent tensile strength of the gneiss first decreased and then increased with anisotropic angle β , rather than the widely reported monotonic increase or single-shoulder response. The shear sliding of biotite along cleavage planes caused stress concentrations in the surrounding brittle minerals, inducing cracks and reducing the overall tensile strength of the gneiss. Micro-observations of the relationship between cracks and biotite orientation identified three types of biotite crystal fragmentation: tensile fragmentation along cleavage planes, shear fragmentation along cleavage planes, and tensile fragmentation perpendicular to cleavage planes. Tensile and shear fragmentation of biotite along cleavage planes negatively affected the gneiss' macroscopic tensile strength. Conversely, when the tensile stress was parallel to the biotite cleavage planes, tensile fragmentation perpendicular to the cleavage planes increased the macroscopic tensile strength of the gneiss due to plastic deformation and high surface energy. Quantitative analysis of shear stress along biotite cleavage planes revealed the mechanical mechanism behind the reduced tensile strength of the East Himalayan gneisses near an anisotropic angle of 30°. These results elucidated the influence of grain-level anisotropy on the macroscopic tensile mechanical properties of intact layered rocks.