The frictional strength and stability of biotite under hydrothermal conditions

Biotite is one of the commonest phyllosilicate minerals in nature with low strength and relatively high chemical stability. Investigation on its frictional properties helps to understand the compositional ground of weak faults. We carried out frictional experiments of biotite gouge at effective normal stress of 200 MPa, pore pressure of 10 MPa and 30 MPa, and temperatures of 300 degrees C and 400 degrees C which correspond to the temperature range of brittle-plastic transition zone. The main results obtained in this work are frictional coefficient and velocity dependence. The coefficient of friction of biotite is about 0. 36, significantly lower than the muscovite case. The mechanical data show weak velocity weakening at 300 degrees C, whereas stick-slips occurred at 400 degrees C indicating stronger velocity weakening. Microstructures of deformed samples show both brittle shear bands and kink bands due to plastic glide in the basal plane, but evidently the mechanism that controls the macroscopic behavior is sliding along the brittle shear bands. The results in this study may help understand the possibility of earthquake genesis in a brittle-plastic transition zone and the mechanical behavior of weak fault zones in the continental crust.