Crystallographic orientations of quartz grain-boundary segments formed during dynamic recrystallization and subsequent annealing

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.