Atomistic origins of mechanical amorphization of SiO2 using solid-state 29Si NMR and local electronic structures for SiO2 high pressure phases using ab initio calculation

Abstract

Silica (SiO2), the most abundant material on the earth, is a major component of the crust and mantle. Therefore the structure of silica affects the dynamic process of crust or the structural properties of the earth interior. The structure of silica changes not only by pressure-temperature change but also mechanical energy like friction without melting. To understand the properties of interior of rocky planets and the geological process, the detailed understanding of the structure of silica with various conditions is needed. In this study, we performed two major subject related to structure of silica. The first study is about atomistic origin of the mechanical amorphization of silica with an experimental methodology. There have been reported the reduction of fault strength during fault slip, and one of the causes is reported as the formation of the amorphous silica and the silica gel layer on the fault plane. The formation of amorphous silica is due to the frictional energy between the fault surfaces, and the formation of silica gel layer is due to reaction with the water present in the layer or air. In previous, studies on frictional experiments using quartzite rocks, reported the formation of silica and silica gel and also the decrease of friction coefficient, and the observation of the silica gel layer in natural faults has been reported. However, due to the complex structure of amorphous silica and the analytical method limitations, the atomic structure of the amorphized silica and the detailed origin of the mechanical amorphization process have not been clearly identified. In this study, silica was amorphized with mechanical energy by ball mill method, and the resulting amorphous structure was analyzed by high resolution nuclear magnetic resonance (NMR) spectroscopy. The morphology of the amorphized silica and the phases formed by other elements were analyzed using XRD, HR-TEM, and EDS-mapping method. Solid-state nuclear magnetic resonance spectroscopy (NMR) is suitable for the analysis of complex amorphous structures because it yields atomic environments in short range order around specific atoms and provides quantitative information on atomic unit bonding. The 29Si MAS NMR spectra of amorphous silica milled at different rates present that the spectra of samples milled above 600 rpm show a broad amorphous peak over -80 to -120 ppm. These amorphous peaks indicate the presence of Q2 and Q3 structures in the mechanically amorphized silica. The results indicate that the mechanical amorphization of silica occurs only above a certain energy level, and that the amorphization process results in a change in the short range atomic structure and imply the presence of reaction with other elements. This results help to understand the atomic structure of the mechanically amorphized silica and the mechanical amorphization process that occurs without melting in the fault plane. The second study is an electronic structure and detailed origin of spectral feature of O K-edge XRS for crystalline silica using computational computation. Crystalline silica undergoes various phase transition according to pressure-temperature change. Therefore, many studies on the structure of high-pressure phase silica have been reported to understand the internal structure of rocky planets in high temperature and high pressure environment. The most powerful method of in situ high-pressure study for electronic structure is O K-edge x-ray Raman spectroscopy (XRS) using diamond anvil cell and as experimental limitation above ~70 GPa, the computational methods is used to understand the XRS spectrum. However, the detailed relation between spectral feature of O K-edge XRS and structure is still in debate. In this study, we calculated electronic structures and O K-edge XRS spectra for various crystalline silica using ab initio calculation, and proved the origin of O K-edge XRS spectrum. Previous studies have suggested that the origin of the O K-edge XRS spectrum is attributed to the number of Si atoms or the number of O atoms. However, in this study, we have found that the O K-edge XRS spectra of hp-cristobalite with 4-coordination Si, of penta-SiO2 with 5-coordination Si and of stishovite with 6-coordination Si are similar, and O site resolved K-edge XRS spectra the penta-SiO2 revealed that the coordination of atoms does not directly affect the O K-edge XRS spectral feature. These results not only provide an understanding of the electronic structure of high-pressure phase silica, but also provide clear criteria for O K-edge XRS spectral analysis.