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

Abstract

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.

Effect of normal stress on the friction of brucite: Application to slow earthquake in the mantle wedge

“…3) and consistent with previous experimental results, that is, 0.38-0.46 and 0.28 for dry and wet brucite at an applied normal stress of 100 MPa at room temperature, respectively Lockner, 2004, 2007). The friction coefficient for dry experiment is also close to the theoretical value of 0.30(3) (Okuda et al, 2019).…”

Atomic-scale interlayer friction of gibbsite is lower than brucite due to interactions of hydroxyls

“…We prepared a supercell that includes two Al-dioctahedral sheets and computed the interlayer energy E ad (x,z) at various displacements x of top sheet in the ab plane (~0.3 Å-mesh grid) and various interlayer distances z (0.05 Å interval; Figure 2a). This study only focuses on two layers adjacent to the shear plane because the difference in the shear stress between two and four-layers simulations is less than 1.2% for brucite (Okuda et al 2019). Here, E ad (x,z) is defined as:…”

“…The calculated PESs for gibbsite and brucite are shown in Figure 4. The potential energies for brucite were calculated in our previous study (Okuda et al 2019).…”

“…For brucite (Okuda et al 2019), the atomic-scale shear stress τ atom exhibited little sliding direction dependence (gray line in Figure 5b). For gibbsite, a clear sliding direction dependence was observed.…”

“…http://www.minsocam.org/ Always consult and cite the final, published document. See http:/www.minsocam.org or GeoscienceWorld Okuda et al 2019). These studies are hereinafter referred to as SKKS18, SKK20, SLSD22, and OKS19, respectively.…”