2.3. Electron Back Scatter Diffraction: acquisition and processing

Electron Back Scatter Diffraction (EBSD) mapping was carried out at the Electron Microscopy Centre of Plymouth University with a (1) JEOL 6610 LV SEM equipped with a NordLys Nano EBSD detector and a (2) JEOL 7001 FEG SEM equipped with a NordLys Max EBSD detector. The analytical configuration for EBSD mapping was 20–25-mm working distance, 70° of sample tilt with respect to the horizontal and 20 KeV of accelerating voltage. The sample symmetry used was monoclinic, and quartz was the only phase indexed, using trigonal crystal system (Laue group 3/m). EBSD patterns were automatically detected and indexed with AZTec software (Oxford Instruments). Map 1 is a 414 × 310 pixel map acquired with a step of 2.4 μm, Map 2 has a size of 1237 × 927 pixels and was acquired with a step size of 1.1 μm, and Map 3 is 585 × 438 pixels and has a step of 1.3 μm.

Noise reduction, following Bestmann and Prior [3], was performed using HKL CHANNEL 5 software by Oxford Instruments, as following: (1) wild spikes, resulting from misindexing problems (pixels with exotic orientation with respect to the surrounding crystal lattice) were corrected by extrapolation (2) zero solution pixels (i.e. pixels that were not indexed); are corrected by extrapolating their orientation from the neighboring pixels and then by iteration (progressively increasing the filtering level from 8 to 5 in Channel 5).

High-angle boundaries were defined at a critical misorientation of 10° and above, allowing grain boundary completion down to 0°, while low-angle boundaries were set for a critical misorientation of 2–10°. The use of a higher symmetry for quartz (3/m instead of 32 Laue group) allows to recognize Dauphiné twin boundaries (twinning between right-handed and left-handed quartz) as grain boundaries with 60° ± 2° of misorientation and <c> as misorientation axis and to disregard them from the grain detection routine. The grain size detection routine of Channel 5 (Tango software) recalculates grain diameters in μm from equivalent-area circles (μm²). Grains measuring less than 3 times the step size (i.e., containing less than 4–9 pixels) were nullified, as they could represent artifacts generated by the noise reduction routine. Grain size statistics of the processed dataset are available in Table 1.

Grain size statistics of EBSD areas. EX: average, expectation; D²X: variance, dispersion; σ: standard deviation; σ/EX: coefficient of variation; Xmin: minimum equivalent diameter value (μm); Xmax: maximum equivalent diameter value (μm); N: number of detected grains.