Mapping intragranular microstructures in quartz: the significance of Dauphiné twinning

Abstract

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, EBSD-based microstructural maps are presented of subgrain and Dauphine twin boundaries in undeformed sedimentary quartzite. ‘Plane Matching’ analysis permits determination of complete orientation of boundaries. A workflow is presented to facilitate necessary crystallographic calculations. 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 subparallel to grain boundary stress concentrations; rotation axes are oriented sub-parallel to bedding dip (017°/10°E). Most exhibit bi-direction boundary migration vectors parallel to bedding normal. EBSD misorientation analysis results are different as they only recognise parallelism of adjacent crystal lattices. Differences are especially apparent for Dauphine 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 Young's modulus and driving forces do not need to be large, explaining the propensity for twinning in many quartzites. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6710513