Mapping intragranular microstructures in quartz: the significance of Dauphiné twinning

Mapping on the microstructural scale can contribute significantly to conventional field and larger scale mapping and understanding of spatial, temporal and process-oriented relationships. Here, electron backscattered diffraction (EBSD)-based microstructural maps are presented of subgrain and Dauphiné twin boundaries in undeformed sedimentary quartzite. ‘Plane matching’ analysis permits determination of the complete orientation of boundaries. A workflow is presented to facilitate the necessary crystallographic calculations. The resulting maps indicate: (1) boundary plane rotation angle/axis pairs, including tilt–twist components; (2) boundary migration vectors; and (3) conventional EBSD misorientation angle/axis pairs. Subgrain boundaries are general with small misorientations and boundary plane normal directions sub-parallel to grain boundary stress concentrations; rotation axes are oriented sub-parallel to the bedding dip (017°/10°E). Most exhibit bi-direction boundary migration vectors parallel to bedding normal. The EBSD misorientation analysis results are different as they only recognize the parallelism of adjacent crystal lattices. The differences are especially apparent for Dauphiné twin boundaries. Maps are presented of twin boundaries using matched plane analysis, including explanation for lateral twin migration. Driving forces to move twin boundaries are also estimated by mapping variations in Young's modulus between parents and twins; differences are significant, indicating that the Young's modulus and driving forces do not need to be large, explaining the propensity for twinning in many quartzites.