Deformation and transformation in experimentally shock-loaded quartz

Abstract

Single crystals of quartz, shock-loaded along the a axis to pressures of 22 Gpa, 24 GPa, 26 GPa and 30 GPa were examined by high-voltage transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction. Asymmetric broadenings of X-ray lines indicate spatial inhomogeneity of shock effects. X-ray streaking angles in the reciprocal lattice planes h0$\bar h$ l, 0k $\bar k$ l and hki0 indicate a slight tilting deformation by rotation about [00.1] in (0001). TEM reveals glass lamellae which are mostly in (01$\bar 1$2) orientation, and are correlated with optical planar elements and with surface steps seen in SEM. No dislocations are found. There are (0001) lamellar features, probably Brazil twins. The (01$\bar 1$2) glass lamellae develop directly from bands of quartz in which intense deformation has produced a fine-scale lamellar to blocky structure, possibly also originating by twinning. Relics of crystalline structure are found in almost completely vitrified lamellae. Stishovite occurs in heavily deformed parts of the 22 GPa and 24 GPa specimens, in patches of densified glass distinct from the sharply bounded lamellae. The nucleationless, pervasive transformation of lamellae to glass, with preservation of their sharp boundaries, is attributed to defect coalescence analogous to vitrification by radiation damage (metamictization). Some patchy glass may be due to melting.