First-Principles Study On Microstructures And Elastic Properties Of Clay Minerals

Clay minerals are the main mineral component of shales, and their elastic properties and mechanical behavior are closely associated with exploration and production of shale gas. In addition, the micro mechanical features of clay minerals have received extensive attention for clay minerals play a important role in nano-composite field. The properties of clay minerals are determined by their characteristic structures, so obtaining structure information of clay minerals accurately and analyzing the relationship between structural characteristics and mechanical properties have great significance for solving the related mechanical problems in their application fields. Very fine particle size and complexities of the compositions and structures make it rather difficult to determine the structures and properties of clay minerals by experiments. On the other hand, flourishing development of theoretical calculation methods provides an effective approach to research this subject. Among many theoretical calculation methods, density functional theory(DFT) has become one of the dominant methods for its high precision and reliability. However, for its own limitation DFT cannot describe non-bonding interactions between crystal layers of clay minerals accurately, which can be successfully solved by introducing the dispersion corrections into DFT. In the present work first-principles calculations based on dispersion-corrected density functional theory(DFT-D2) were performed to study the microstructures and elastic properties of kaolinite, pyrophyllite and montmorillonite, which are typical clay minerals, and determine the effect of structural characteristics on their elastic properties.1.The microstructures and electronic properties of kaolinite and pyrophyllite were investigated by using first-principles calculations based on DFT-D2, and how the composition and bonding means of structure layers influence their microstructures were analyzed. The results showed that using DFT-D2 method could dramatically improve the predictive values of lattice parameters, which were more close to the experimental data than those by using standard DFT method. It can be found that the composition and bonding means of structure layers affect obviously the bond length and bond angle of silicon-oxygen bond, the sharing-edge bond angle in octahedral layer and the orientation of hydroxyl group by comparing the structural features of kaolinite and pyrophyllite. Moreover, the hydrogen bond between crystal layers of kaolinite not only led to far shorter interlayer thickness than pyrophyllite, but also restrained the distortion of tetrahedral layers. In addition, the electronic properties also were influenced by the structure. It indicated that the band gap of kaolinite was more narrow than pyrophyllite and the negative charge of apical oxygen in kaolinite was less than that in pyrophyllite.2. For two typical mode of isomorphic substitutions in montmorillonite(octahedral Al3+/Mg2+ substitution and tetrahedral Si4+/Al3+ substitution), the crystal structures and electronic properties were changed when different counterions intercalated in the interlayer of montmorillonite. It suggested that the distribution of isomorphic substitutions has little influence on the lattice parameters, and the structures with more distributed substitutions were more stable energetically. However, the interlayer thickness, the locations of counterions, the charge distribution and the band gap width varied with the substitution mode when the same counterions are in the interlayer. For two mode of isomorphic substitutions, the size of counterion is the important factor that affects the microstructure of montmorillonite. With the radius of ions increasing, not only the interlayer thickness increased by a quadratic function manner, but also the lattice angles????? and the layer thickness(the tetrahedral sheet thickness and the octahedral sheet thickness) changed. Besides that, the extent of tetrahedral sheet distortions decreased with the increasing size of counterion. The counterions were located in a ditrigonal ring close to the substitution site and the larger counterions were close to the middle of interlayer.3. By first-principles calculations based on DFT-D2, the elastic properties of kaolinite and pyrophyllite and the variation of their structures and elastic properties under different hydrostatic pressures from 0GPa to 20 GPa were investigated. The calculated results showed that for the difference of their structures the stiffness parallel to the crystal layer of kaolinite was slightly less than that of pyrophyllite, but the stiffness perpendicular to the crystal layer of kaolinite was significantly larger than that of pyrophyllite. In the two kinds of minerals, pyrophyllite had a stronger ability of resistance to volume change and kaolinite had a stronger ability of resistance to deformation. During the pressurization process the volumes of two minerals varied in a quadratic function manner and the extent of compression was similar with pressure increasing, but the tetrahedral distortion of kaolinite was significantly greater than that of pyrophyllite; the elastic constants of these two minerals increased with pressure increasing, especially along the stacking direction. At the late stage of compression(10~20GPa), the average elastic properties of polycrystals of kaolinite and pyrophyllite had a similar variation rule, but the elastic modulus and the wave velocity of pyrophyllite were larger than that of kaolinite under the same pressure.4. For two typical mode of isomorphic substitutions, the elastic properties of montmorillonite with different counterions were investigated. The study shows that the intercalation of counterions strengthens the correlation of deformation between the crystal layers and the orientation perpendicular to the crystal layers, and the intercalation of medium-sized counterions could improve the stiffness and the deformation correlation along the orientations paralleling to the crystal layers. For the same counterion, in octahedral substitution model the elastic constants(C11, C22, C33, C12) and Young’s modulus were greater than these of tetrahedral substitution models; and C55, C13 and C23 is slightly smaller than these of tetrahedral substitution models. The results of average elastic properties show that montmorillonite polycrystals with smaller counterions had higher elastic modulus and wave velocities.