[SCG54-P01] First-principles study on frictional characteristics of gibbsite
Keywords:Sheet-structure minerals, Crystal structure, Frictional characteristics, Gibbsite, First-principles calculation
In this study, we focus on the frictional characteristics of two sheet-structure minerals, brucite (magnesium hydroxides) and gibbsite (aluminum hydroxides). Brucite has the friction coefficient of 0.42 and shows stick-slip behavior, whereas gibbsite has the friction coefficient of 0.75 and shows stable sliding (Moore & Lockner, 2004). These two minerals were composed by MgO6 and AlO6 octahedral sheets, respectively (Fig. 1). The difference between them was brucite has no vacant site within the octahedral sheet, whereas gibbsite has 1/3 vacancy with in the sheet (Fig. 2). These two octahedral sheets were the main component of phyllosilicates and their influence of crystal structures on frictional characteristics is important for understanding the frictional characteristics of sheet-structure minerals. We theoretically examined the frictional characteristics of brucite in our previous study (Okuda et al., submitted). Here we calculate atomic-scale frictional characteristics of gibbsite using first-principles calculations based on the density functional theory and compare the results of gibbsite with those of brucite.
Preliminary results showed that gibbsite has lower atomic-scale shear stress than brucite. This is because the hydroxyls of gibbsite can be easily moved due to the vacancy within the octahedral sheet of gibbsite and that makes the potential energy low and increases the structural stability while being sheared. On the other hand, the yield strength of gibbsite is 1~2 GPa estimated by Mohs hardness (Mehrotra et al., 2015; Broz et al., 2006). This yield strength is lower then that of brucite (4.0 GPa (Okuda et al., submitted)) and this low yield strength increases the macroscopic friction coefficient when considering the adhesion theory of friction. We will quantitatively discuss the atomic-scale shear stress and yield strength of gibbsite and investigate what controls the difference of frictional characteristics between brucite and gibbsite.