EBSD-based criteria for coesite-to-quartz transformation: sample 11-01 (Tso Morari, Himalaya) and DW09-52 (Dora Maira, Alps) datasets

The quartz crystal preferred orientations (CPO) were determined using EBSD collected on an FEI Quanta 650 FEG E-Scanning electron microscope equipped with Oxford Instruments Aztec (version 3.3) acquisition software and a NordlysNano EBSD camera. The sample was mapped at low vacuum (50 Pa, H2O), tilted to 70°, with an accelerating voltage of 30 kV and a working distance of 15 mm. In order to reduce mapping times, step sizes of 20 μm were used and Kikuchi patterns were binned (2 x 2 binning).
EBSD data were processed using the MTEX package for MATLAB (Bachmann et al., 2010; Mainprice et al., 2014), which was also used to create and analyse crystal orientation pole figures as well as All-Euler and band contrast maps. Isolated pixels and small grains ≤ 5 pixels were removed. Non-indexed points surrounded by ≥ 6 indexed points were given the average orientation of their neighbours using the smoothing spline filter.
Crystal axes and poles to crystal planes are plotted on pole figures with lower hemisphere projections and contoured in units of multiples of uniform distribution (m.u.d.) using an orientation density function. Quartz pole figures were constructed for < a > and [c] crystallographic axes, and {r} and {z} poles to planes. The < a > axes were plotted as antipodal projections to avoid being plotted on opposite hemispheres. Subsets of quartz crystallographic orientation data with common orientations were extracted from each dataset by selecting orientations within 5° of statistically defined modal orientations.