Interlayer energy of pyrophyllite: Implications for macroscopic friction
Interlayer energy of pyrophyllite: Implications for macroscopic friction
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DOI：
10.2138/am-2020-7333
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发表时间：
2020-08
影响因子：
3.1
通讯作者：
H. Sakuma;K. Kawai;T. Kogure
中科院分区：
文献类型：
--
作者：
H. Sakuma;K. Kawai;T. Kogure
摘要层状硅酸盐的变形可以控制地壳的动力学。对于一些典型的层状硅酸盐来说，应力和变形之间的现象学关系是已知的；然而，源于晶体结构的潜在物理机制却鲜为人知。通过密度泛函理论计算和原子尺度摩擦理论计算，揭示了叶蜡石沿基面的变形机制。层间滑动形成的稳定和亚稳的层间结构与高分辨电子显微镜的实验结果一致。稳定层间结构和亚稳态层间结构之间的势能差异可以解释为相邻层之间双八面体薄片堆积的差异。估算的叶蜡石相邻层间的摩擦系数与原子力显微镜的结果一致，表明该方法可以很好地估计原子尺度的摩擦。在我们的模拟中计算的剪应力与法向应力呈线性关系，并且与沿基面的滑动方向没有明显的晶体相关性。层间摩擦的结晶学各向同性是由于叶蜡石中没有层间阳离子，而白云母则表现出以往研究中观察到的结晶学各向异性。利用接触面积从原子尺度的摩擦力估算了叶蜡石单晶的宏观摩擦力。理想层间滑动的宏观摩擦系数估计为0.134，小于湿多晶泥层剪切实验中报道的值0.276。这种差异主要可以用叶蜡石颗粒在断层泥中的取向度来解释。用叶蜡石无定向层的简单模型估算的摩擦系数为0.203±0.001，与文献报道的0.276相近，明显小于由经验比尔利定律估算的常见矿物的摩擦系数(0.276-0.85)。这些结果表明，层状硅酸盐的弱层间摩擦对天然断层中泥层的低摩擦强度有很大的影响。我们的方法和结果对于理解层状硅酸盐断层泥的唯象摩擦定律背后的物理原理是有用的。
Abstract Deformation of phyllosilicate can control the dynamics of the Earth’s crust. The phenomenological relationship between stress and deformation is known for some typical phyllosilicates; however, the underlying physics originating from the crystal structures is poorly understood. In this study, the deformation mechanism of pyrophyllite along basal planes was revealed through density functional theory calculations and atomic-scale theory of friction. The stable and metastable interlayer structures formed by interlayer slide were consistent with the experimental results reported previously by high-resolution transmission electron microscopy. The difference in potential energies between stable and metastable interlayer structures can be interpreted as the diference in the stacking of dioctahedral sheets between the adjacent layers. The estimated friction coefficient of the pyrophyllite between adjacent layers was consistent with the results of atomic force microscopy, suggesting that atomic-scale friction can be adequately estimated by this method. The calculated shear stress in our simulations has a linear relationship with the normal stress and has no significant crystallographic dependence on sliding direction along the basal planes. The crystallographic isotropy of interlayer friction is explained by the absence of interlayer cations in pyrophyllite, while muscovite showed crystallographic anisotropy as observed in previous studies. The macroscopic friction of a single crystal of pyrophyllite was estimated from atomic-scale friction by using the area of contact. The macroscopic friction coefficient of ideal interlayer sliding was estimated to be 0.134, which was smaller than a reported value (0.276) in shear experiments conducted for wet polycrystalline gouge layers. This diference can be primarily explained by the degree of orientation of pyrophyllite particles in the gouge layers. The friction coeficient estimated by a simple model of randomly oriented pyrophyllite gouge layer was 0.203 ± 0.001, which was similar to the reported value of 0.276 and clearly smaller than the values (0.6–0.85) of common minerals estimated by the empirical Byerlee’s law. These results indicate that weak interlayer friction of phyllosilicates has a large effect on the low frictional strength of gouge layers in natural faults. Our methodology and results are useful for understanding the physics behind the phenomenological friction laws of phyllosilicate gouge.