Development of crystallographic preferred orientations by nucleation and growth of new grains in experimentally deformed quartz single crystals

Abstract

Deformation experiments have been carried out to investigate the effect of dynamic recrystallisation on crystallographic preferred orientation (CPO) development. Cylindrical samples of natural single crystals of quartz were axially deformed together with 1 vol.% of added water and 20 mg of Mn 2O 3 powder in a Griggs solid medium deformation apparatus in different crystallographic orientations with compression direction: (i) parallel to < c>, (ii) at 45° to < c> and 45° to < a> and (iii) parallel to < a>. The experiments were performed at a temperature of 800 °C, a confining pressure of 1.2 GPa, a strain rate of ∼ 10 − 6 s − 1 , to bulk finite strains of ∼ 14–36%. The deformed samples were analysed in detail using optical microscopy, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Two different microstructural domains were distinguished in the deformed samples: (i) domains with undulatory extinction and deformation lamellae, and (ii) domains with new recrystallised grains. Within the domains of undulatory extinction, crystal-plastic deformation caused gradual rotations of the crystal lattice up to ∼ 30° away from the host orientation. New recrystallised grains show a strong CPO with c-axis maxima at ∼ 45° to the compression direction. This is the case in all experiments, irrespective of the initial crystallographic orientation. The results show that c-axes are not continuously rotated towards the new maxima. The new grains thus developed through a mechanism different from subgrain rotation recrystallisation. New grains have a subeuhedral shape and numerous microcavities, voids, fluid channels and fluid inclusions at their grain boundaries. No host control is recorded in misorientation axes across their large angle grain boundaries. New grains might have been created by nucleation from solution in the μm-scale voids and microfractures. The CPO most likely developed due to preferred growth of the freshly precipitated grains with orientations suitable for intracrystalline deformation at the imposed experimental conditions.