First-principles Investigation of Frictional Characteristics of Brucite: An Application to Its Macroscopic Frictional Characteristics

Because sheet-structure minerals are often observed in natural faults and have lower friction coefficients than the other common rocks and rock-forming minerals, it has been suggested that their frictional characteristics are related to fault strength. Despite their importance for understanding fault dynamics, what controls their frictional characteristics has not been clarified yet. Because the friction coefficients of these minerals depend on the mineral species, the atomic-scale crystal structure can be a clue for understanding the mechanism. In this study, the variation of potential energy on the brucite (0001) shear plane (potential energy surface, PES) was calculated by using first-principles calculations based on density functional theory. The shear stress along the sliding path is calculated by computing the displacement derivative of the PES. According to the adhesion theory of friction, we found that the atomic-scale frictional parameters obtained by using the PES could explain the macroscopic friction coefficients when taking the experimental indentation strength into account. Then, we propose how to estimate the constitutive parameter a of the rate- and state-dependent friction law from the PES of sheet-structure minerals. The estimated value is consistent with the typical range for minerals investigated by previous experiments. The difference in the PES among sheet-structure minerals can therefore be a key property to explain their frictional characteristics such as the tendency for frictional instability of natural faults.