What is Friction Angle?
For a planar, clean (no infilling) fracture, the cohesion is zero and the shear strength is defined solely by the friction angle. The friction angle of the rock material is related to the size and shape of the grains exposed on the fracture surface. Thus, a fine-grained rock and rock with a high mica content tend to have a low friction angle, whereas coarse-grained rock has a high friction angle.
Angle of Internal Friction