Crystallographic Control on the Development of Texture in Precipitated Quartz Grains
In this study we analysed microstructures and determined [c]-axis textures of quartz crystals in veins formed parallel to composition banding in naturally deformed iron oxide-quartz rocks. Only veins of few millimeters thick were sampled. These veins were formed in a regime of non-coaxial deformation under temperature of ~300°C. We made thin sections from rock slabs cut perpendicular to shear plane and parallel to shear direction. In thin sections veins are composed of large single quartz crystals of lens or rhomb-shaped blocks similar to s-porphyroclast systems. Lattice distortion (i.e. undulose extinction, gradual lattice banding and subgrain boundaries) occurs in single crystals as revealed by optical microscopy. Distortion was caused by slip of dislocations preferentially on basal planes. These are also planes along which microcracks developed. Distinct types of microcracks are individualized based on size, orientation and distribution of voids. Microcracrack voids are filled by polycrystalline quartz aggregates. In contrast to single crystals, these aggregates do not have any optical microstructure that might be related to crystal plastic process. Moreover grain size distribution are quite different from those related to dynamic recrystallized aggregates. Despite of that, polycrystalline quartz aggregates have strong [c]-axis preferred orientations. These orientations are similar to those of single crystals close to the microfracture walls. In large spaced voids c-axes orientation of quartz in polycrystalline aggregate have significant misorientation angles with respect to the single crystal [c]-axis orientation, reaching values up to 45° to the foliation plane (XY section of the finite strain). Based on microstructural and textural data we propose a mode for quartz [c]-axis texture development in both single crystals and polycrystalline aggregates that fill microcrack voids.
Paul Van Houtte and Leo Kestens
L. Lagoeiro et al., "Crystallographic Control on the Development of Texture in Precipitated Quartz Grains", Materials Science Forum, Vols. 495-497, pp. 57-62, 2005