Experimental Evidence of Basal Slip in Quartz

Abstract

Polished crystals of quartz, deformed at high temperature and confining pressure with high shear stress on the basal plane, develop slip bands on their surfaces parallel to the trace of the basal planes, indicating slip on this plane. The distribution of the slip markings on cylindrical surfaces shows that the a-axes are the slip directions. Thin sections of the polished crystals contain deformation lamellae parallel to the base. The deformation lamellae are therefore parallel to slip zones in these crystals. Optical studies of basal deformation lamellae with phase-contrast illumination indicate that they are sharp boundaries, less than 0.2 micron thick, separating regions with higher and lower refractive indices than the quartz at some distance from the lamellae. Compensator measurements also reveal birefringences higher and lower than normal on opposite sides of these boundaries. Changes of indices and birefringence are consistent with the photoelastic effects which would be expected from an array of basal edge dislocations with Burgers vector parallel to an a-axis. Calculations of the stress field and resultant stress-optical effects due to such an array require a density of dislocations of per centimeter in average lamellae. The validity of this model is supported by electron micrographs of etched polished surfaces of crystals containing experimentally produced lamellae. The lamellae appear as rows of pyramidal etch pits which are exactly linear within 50 Å. The density of pits ranges from 5 to per centimeter. The geometrical characteristics of deformation bands parallel to the c-axis indicate that they are kink bands originating by basal slip; their orientations relative to the a-axes support the conclusion that the a-axes are the slip directions. The band boundaries are presumed to consist of "walls" of locked basal edge dislocations. This model is consistent with the lack of measurable optical changes in the vicinity of most band boundaries; changes of indices and birefringence along some band boundaries, qualitatively similar to lamellae, result from residual elastic stresses at asymmetrical boundaries. These elastic stresses are commonly relieved by tensile fracture in the undeformed host crystal and may be partially relieved by the development of prismatic dislocations at the kink-band boundary. Most natural lamellae so far studied exhibit the appearance in phase-contrast illumination and the changes in birefringence which have been found in experimental lamellae. Electron microscopy of etched polished thin sections of natural quartzite containing lamellae shows bands of etch pits coincident with the lamellae. The distribution of etch pits within these lamellar bands is much less regular than in experimental lamellae, but the density of etch pits is of the order required to give the observed optical effect if the etch pits represent dislocations predominantly of the correct sign. It is concluded that natural lamellae are typically comprised of irregular arrays of locked-in dislocations. Differences of orientation between natural and experimental near-basal lamellae are discussed; and it is concluded that natural lamellae may be initially parallel to the base, with subsequent internal rotation by other slip systems, or they may originate at an angle to the base as en échelon arrays of basal dislocations.