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

The crystallographic preferred orientation of fibrous quartz veins deformed under subgreenschist facies conditions along a temperature gradient from 2 70 degrees to 370 degrees C has been investigated. To explore the active me chanism during deformation of the quartz fibres, the complete crystallograp hic orientation of individual recrystallized grains and subgrains was deter mined by using a combination of CIP (computer-integrated polarization micro scopy), EBSD (electron back-scatter diffraction), and additional specially developed image analysis techniques. It was found that the quartz fibres de formed along crystallographically controlled shear bands. From inverse pole figures of normals to the shear band boundaries it is evident that these b ands occur along the positive and negative rhomb planes of quartz. It is su ggested that they are initiated as fractures along these planes. Inside the shear bands, small grains broken off from the host grain show passive rota tion of the lattice, consistent with the shear sense of the shear bands. Co ntinued deformation and increasing temperature lead to different recrystall ization microstructures inside and outside the shear bands and to a partial obliteration of the sharp orientation relationships. (C) 1999 Elsevier Sci ence B.V. AU rights reserved.