The roles of crystallographic orientation and crystal origin in ferrobielastic twinning of quartz under uniaxial stress

摘要

Ferrobielastic twinning in quartz single crystals has been investigated in relation to crystal-lographic orientation and crystal origin. By means of uniaxial compressive stress experiments, complete volumetric transformation to the alternate Dauphine twin state was performed in X- and AT-cut samples of natural and synthetic quartz. Either polarized light or synchrotron radiation X-ray topography was employed to observe the movement of twin boundaries. The dependence of the coercive stress and the preferential orientation of twin boundaries on the crystallographic orientation are explained in terms of the thermodynamic potential of the Gibbs free energy. In relation to natural crystals, the stress levels related to optical visualization and complete transformation to the alternate twin state were 50% and 30% lower for synthetic quartz. These results are qualitatively discussed by the pinning of Dauphine twin boundaries at randomly distributed impurity-related point defects which were evaluated by infrared spectroscopy and ion coupled plasma spectrometry. The spontaneous irreversibility of ferrobielastic twinning on unloading is confirmed.