The electron backscatter diffraction (EBSD) analysis procedure for crystal preferred orientations to reduce loss of EBSD map data: An example of antigorite

Abstract

Indexing Kikuchi or electron backscatter diffraction (EBSD) patterns from rock samples provides information on mineral phases and their crystal orientation and maps of this information are widely used for mineral microstructural analysis. However, uncertainties in the results due to possible problems of mis-indexing have not been closely studied. One approach to identify unreliable data is to use a filter based on maximum mean angular deviation (MAD) values. Mechanically weak minerals, in particular hydrous sheet silicates, are particularly challenging to analyze due to surface damage formed during sample preparation and electron beam irradiation. The application of stringent MAD filters to extract high-quality accurate data for minerals such as antigorite drastically reduces the proportion of data that can be used for analysis in particular in surfaces cut perpendicular to rock foliation. Combining the MAD data with the number of the EBSD Kikuchi diffraction bands provides a new improved protocol to filter EBSD data that identifies a higher proportion of high-quality data (up to > ∼50%) compared to the conventional method using only MAD (∼5–29%), even from sample surfaces normal to the foliation, while maintaining the same accuracy as the conventional method. Use of the improved protocol will enable more reliable EBSD data sets to be provided which will improve any associated microstructural analyses. Previously reported crystallographic pole figure (CPO) patterns for hydrous sheet minerals such as antigorite may have been calculated from data sets that include a significant proportion of incorrect crystal orientations. Use of the approach proposed in this study may improve our understanding of rock structure, in particular where hydrous minerals are important and hence rheological studies of hydrous shear such as those present along subduction interfaces.