Orientation analysis of localized shear deformation in quartz fibres at the brittle–ductile transition

Abstract

The crystallographic preferred orientation of fibrous quartz veins deformed under subgreenschist facies conditions along a temperature gradient from 270° to 370°C has been investigated. To explore the active mechanism during deformation of the quartz fibres, the complete crystallographic orientation of individual recrystallized grains and subgrains was determined by using a combination of CIP (computer-integrated polarization microscopy), EBSD (electron back-scatter diffraction), and additional specially developed image analysis techniques. It was found that the quartz fibres deformed along crystallographically controlled shear bands. From inverse pole figures of normals to the shear band boundaries it is evident that these bands occur along the positive and negative rhomb planes of quartz. It is suggested that they are initiated as fractures along these planes. Inside the shear bands, small grains broken off from the host grain show passive rotation of the lattice, consistent with the shear sense of the shear bands. Continued deformation and increasing temperature lead to different recrystallization microstructures inside and outside the shear bands and to a partial obliteration of the sharp orientation relationships.