The crystallographic orientations of two sets of 3144 and 1660 straight segments of sutured quartz grain boundaries, formed during dynamic recrystallization and subsequent annealing at lower greenschist and lower amphibolite-facies conditions respectively, have been studied by combined universal-stage and electron backscatter diffraction measurements. The segments preferentially occupy orientations of ~ 20–60° to quartz-c and segments sub-parallel to basal and prism planes are under-represented, however, more strongly at amphibolite-facies conditions. Few segments have low-index orientations to both neighbouring grains. Coherent or nearly coherent grain boundaries represent only ~2.4% of all existing grain-boundary orientations, i.e. show random distribution and, consequently, do not represent orientations of relatively low energy. At triple junctions grain-boundary segments opposite to large dihedral angles are preferentially oriented within a ~ 25–65° girdle to quartz-c whereas segments opposite to small dihedral angles preferentially occupy orientations partly sub-parallel to c. This suggests lower energies of segments oriented at ~ 25–65° to quartz-c compared to segments sub-parallel to c.
Measurements on samples with different annealing temperatures show that grain-boundary segments are increasingly concentrated at ~ 20–60° to quartz-c with increasing temperatures. Consequently, the angle between c and grain-boundary planes represents a potential geothermometer.