Electron irradiation induced vitrification at dislocations in quartz
Recent observations using the ‘weak beam’ electron microscopy technique of split images of dislocations in quartz have been interpreted as arising either from dissociation of the dislocations into partials1, or from a glassy (metamict) phase at the core2, resulting from irradiation damage. In this letter we describe observations of lattice images of near end-on dislocations in quartz, which enable us to distinguish between these two models.
A model of hydrolytic weakening in quartz
Hobbs et al. (1972) suggest that the Haasen theory for diamond structure crystals may explain the observed behavior of hydrolytically weakened single crystals of quartz. There is no provision for the effect of varying OH content in the theory, of course, nor does it explain the great work hardening observed in quartz at low temperature or high strain rate. This paper is concerned with extensions to the Haasen theory required to account for the varying shapes of the observed stress-strain curves, the effect of varying water content, and the behavior observed in stress relaxation experiments. The rate-controlled process in hydrolytically facilitated slip is believed to be dislocation growth, which requires either ‘radial’ diffusion of HOH to the newly growing segment or ‘core’ diffusion, in which HOH diffuses parallel to the dislocation line to permit creation of a new segment. Radial diffusion is limited by exhaustion of free HOH as the dislocation density increases. Dislocation velocity proportional to HOH concentration is assumed, as suggested by Blacic. A Haasen-type model incorporating these processes plus recovery fits the observations of Hobbs et al., Balderman and Blacic, and Griggs on crystals of the O+ orientation. The Hobbs et al. observations on ⊥r crystals of varying water content are in conflict with this model. The model predicts the existence of a ‘fundamental strength’ in a temperature-strain rate regime that is easily accessible for experimental verification or disproof.
Computer Simulation of Dislocation Images in Quartz
Transmission electron microscopy (TEM) has become one of the most important tools in the characterization of crystal defects. TEM yields images of individual dislocations, making it invaluable in the determination of slip systems. These images are a result of a diffraction process and as such must be interpreted with considerable care. Since the introduction of the two-beam dynamical theory of electron diffraction for imperfect crystals by Howie and Whelan (1961) many calculations of the contrast produced by various crystal defects have been made and they showed good agreement with experiments. From this theory many general rules for the contrast of various defects have been formed. The invisibility criteria for dislocations in an isotropic crystal (g · b = 0 and g · b × u = 0) can be derived directly from the equations of the dynamical theory. In general, however, such rules cannot be established for more complicated situations. It has been shown, for example (Head et al., 1967; Head, 1967; Humble, 1968), that the above invisibility criteria are not valid for anisotropic crystals. In such cases numerical solutions of the Howie-Whelan equations may be necessary to interpret the contrast arising from the dislocations. Since most rock-forming minerals have compositions, structures, and properties more complicated than the materials (primarily metals) for which many of these general rules have been developed, it is important that theoretical calculations be made before attempting to apply any of them.