Nanoscale visualization of high-angle misorientations in quartz-rich rocks using SEM-EBSD and Atomic Force Microscopy

Abstract

High-angle misorientations can significantly influence material properties. In this study, optical microscopy, Scanning Electron Microscope-Electron Backscatter Diffraction (SEM-EBSD), and Atomic Force Microscopy (AFM) have been used to investigate high-angle misorientations in quartz-bearing crustal rocks. Thin sections of high-grade quartzofeldspathic rocks were subjected to chemical mechanical polishing (CMP) with colloidal silica. In quartz, high-angle misorientations like random high angle grain boundaries (RHAGBs) and Dauphiné twin boundaries (DTBs) could be discriminated using EBSD techniques but not optical microscopy. In nanoscale AFM images, indented channels are observed along RHAGBs but not DTBs; these result from material removal during CMP, indicating lower compactness of RHAGBs compared to DTBs. Along any RHAGB, EBSD reveals different misorientations across segments between consecutive RHAGB-DTB intersections. Grains adjacent to these RHAGB segments have angles between their c-axes varying from 61-66° with parallel {101‾2} planes, and 81–84° with parallel {112‾2} planes, respectively. These symmetries represent the Japan and Sardinian twin laws of quartz, indicating that the RHAGB segments become low-energy twin boundaries, thereby reducing the overall surface energy of the aggregate. Finally, these results suggest that apart from surface topography quantification and high-resolution nanoscale imaging, AFM in conjunction with SEM-EBSD can be used for precisely locating sites for TEM study.