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

Ferrobielastic twinning in quartz single crystals has been investigated in relation to crystallographic orientation and crystal origin. By means of un iaxial compressive stress experiments, complete volumetric transformation t o the alternate Dauphine twin state was performed in X- and AT-cut samples of natural and synthetic quartz. Either polarized light or synchrotron radi ation X-ray topography was employed to observe the movement of twin boundar ies. The dependence of the coercive stress and the preferential orientation of twin boundaries on the crystallographic orientation are explained in te rms of the thermodynamic potential of the Gibbs free energy. In relation to natural crystals, the stress levels related to optical visualization and c omplete transformation to the alternate twin state were 50% and 30% lower f or synthetic quartz. These results are qualitatively discussed by the pinni ng of Dauphine twin boundaries at randomly distributed impurity-related poi nt defects which were evaluated by infrared spectroscopy and ion coupled pl asma spectrometry. The spontaneous irreversibility of ferrobielastic twinni ng on unloading is confirmed.