Dauphiné twin in a deformed quartz: characterization by electron channelling contrast imaging and large-angle convergent-beam diffraction

Abstract

A Dauphiné twin (DT) in a deformed quartz was visualized for the first time by using orientation-optimized electron channelling contrast imaging (ooECCI) under Bragg conditions of the rhombohedral planes. The visualization in backscattered electron (BSE) imaging with a scanning electron microscope (SEM) is possible due to the electron excitations of positive and negative rhombohedral planes from respective twin domains. Those diffraction planes have different structure factors and scattering amplitudes in electron diffraction and are exchanged in a pair of DT domains. The large-angle convergent beam diffraction (LACBED) patterns on the <011‾0> zone axis were displayed with the absence of two-fold symmetry axis along the [0001] direction in an individual twin domain but with a 180° rotation relation along the c direction between a pair of the twin domains. Related to the DT law, the LACBED pattern across a DT boundary showed the higher six-fold rotation symmetry, which is the same as that of the high-temperature β quartz. The off-axis LACBED patterns displaying Bragg lines demonstrated no angular misfit over the twin boundary. This non-orientation misfit on the twin domains allows us to confirm the visualization mechanism of DT in ECCI. The different contrast of a pair of DT domains in the BSE images originates not from a misorientation between the two domains but from different diffraction intensities between positive and negative rhombohedral planes in quartz. The observation procedure from ECCI with SEM to LACBED with TEM (transmission electron microscope) imaging presented here is indispensable for a deep understanding of the role of twinning, ranging spatially from the micrometre scale for a SEM of the bulk specimen to the nanometre scale for a TEM of the interaction with crystal defects in deformation because of their optical invisibility in a conventional petrological microscopy. Following this procedure, <a> dislocations on a rhombohedral plane were characterized in the vicinity of DTs in naturally deformed quartz. This result implies that non-basal slips, e.g. a rhombohedral slip system, i.e. 1/3<1‾1‾20>{101‾1} (<a>{r}) and 1/3<11‾20>{101‾2} (<a>{π}), were activated in the vicinity of DTs. The activity of <a> dislocations on a non-basal plane resulted from the interaction between DTs and dislocations during plastic deformation.