Plastic-deformati on mechanisms in quartz: The effect of water

High-temperature and pressure-deformation experiments indicate that small amounts of water strongly affect the plastic-deformation mechanisms that operate in quartz. Inherently dry natural quartz crystals exhibit a transition from T = (0001), to , t = 0001] glide at approximately 700° C at a strain rate of 8 · 10⁻⁶ sec⁻¹. The transition temperature decreases with decreasing strain rate. When water is diffused into dry natural crystals at high temperature, duplex slip occurs on the systems (0001), 112̄0and either {112̄0}, 0001] or {101̄0}, 0001] depending on which of the latter systems is most highly stressed.Hydrothermally grown synthetic quartz crystals contain relatively large amounts of structurally bound water. In these crystals, a transition occurs from (0001), 112̄0 to dominantly pencil glide in the 0001] and possibly 112̄3 directions. The transition temperature is strongly dependent on the water content of the crystals, and coincides with the hydrolytic weakening temperature defined by Griggs and Blacic for these crystals. Inhomogeneous distribution of water in the synthetic crystals controls the distribution of plastic strain and may, in part, control the orientation of subbasal deformation lamellae in these.It is suggested that anisotropic diffusion of H-OH in quartz may be responsible for the operation of the 0001] pencil-glide system in the hydrolytic weakening regime. Crystal structure anisotropy may also play a role in the slip-system transition through its influence on the Peierls stress, and this is most likely the dominant factor in the dry crystals.