Crystallographic orientations of quartz grain-boundary segments formed during dynamic recrystallization and subsequent annealing
Liebl, Christoph; Kuntcheva, Boriana; Kruhl, Jörn H.; Kunze, Karsten
European Journal of Mineralogy Volume 19 Number 5 (2007), p. 735 - 744
published: Nov 9, 2007
ArtNo. ESP147051905013, Price: 29.00 €
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.