Hans-Rudolf Wenk

Texture and anisotropy

A large number of polycrystalline materials, both manmade and natural, display preferred orientation of crystallites. Such alignment has a profound effect on anisotropy of physical properties. Preferred orientation or texture forms during growth or deformation and is modified during recrystallization or phase transformations and theories exist to predict its origin. Different methods are applied to characterize orientation patterns and determine the orientation distribution, most of them relying on diffraction. Conventionally x-ray polefigure goniometers are used. More recently single orientation measurements are performed with electron microscopes, both SEM and TEM. For special applications, particularly texture analysis at non-ambient conditions, neutron diffraction and synchrotron x-rays have distinct advantages. The review emphasizes such new possibilities. A second section surveys important texture types in a variety of materials with emphasis on technologically important systems and in rocks that contribute to anisotropy in the earth. In the former group are metals, structural ceramics and thin films. Seismic anisotropy is present in the crust (mainly due to phyllosilicate alignment), the upper mantle (olivine), the lower mantle (perovskite and magnesiowuestite) and the inner core (e-iron) and due to alignment by plastic deformation. There is new interest in the texturing of biological materials such as bones and shells. Preferred orientation is not restricted to inorganic substances but is also present in polymers that are not discussed in this review.

Combined texture and structure analysis of deformed limestone from time-of-flight neutron diffraction spectra

The orientation distribution of a textured polycrystalline material has been traditionally determined from a few individual pole figures of lattice planes hkl, measured by x-ray or neutron diffraction. A new method is demonstrated that uses the whole diffraction spectrum, rather than extracted peak intensities, by combining the orientation distribution calculation with the crystallographic Rietveld method. The feasibility of the method is illustrated with time-of-flight neutron diffraction data of experimentally deformed polycrystalline calcite. It is possible to obtain quantitative information on texture, crystal structure, microstructure, and residual stress from highly incomplete pole figures and from regions of the diffraction spectrum containing many overlapping peaks. The approach provides a key for quantitative texture analysis of low symmetry compounds and of composites with complicated diffraction spectra.

Operational texture analysis

Abstract A comprehensive overview is presented of the elements that enter quantitative techniques of texture measurement, analysis and representations. There are many potential errors to be corrected and many choices to be made in all these stages, and we present those that we consider most appropriate. They have been implemented in a software package that is available publicly. A number of novel techniques are used including, for example, in the representation of measured textures, both for quantitative visual inspection and for use in the prediction of anisotropic properties. The symmetry of the test sample is allowed to be general, and that of the crystal structure may be as low as trigonal.

BEARTEX: a Windows-based program system for quantitative texture analysis

BEARTEX is a general PC-based Windows software package for quantitative texture analysis. The 30 programs that it contains provide corrections for experimental pole figures, orientation distribution calculations from complete or incomplete pole figures for all crystal and sample symmetries down to triclinic, graphical display of data, polycrystal tensor property determinations and various other operations.

Superplasticity in Earth's Lower Mantle: Evidence from Seismic Anisotropy and Rock Physics

In contrast to the upper mantle, the lower mantle of the Earth is elastically nearly isotropic, although its dominant constituent mineral [(Mg,Fe)SiO3 perovskite] is highly anisotropic. On the basis of high-temperature experiments on fabric development in an analog CaTiO3 perovskite and the elastic constants of MgSiO3 perovskite, the seismic anisotropy was calculated for the lower mantle. The results show that absence of anisotropy is strong evidence for deformation by superplasticity. In this case, no significant transient creep is expected in the lower mantle and the viscosity of the lower mantle is sensitive to grain size; hence, a reduction in grain size will result in rheological weakening.

Modelling plastic deformation of peridotite with the self‐consistent theory

Theories for deformation of polycrystals have been substantially refined, enabling us to model deformation of metals and minerals with considerable sophistication. So far, most modelling has been confined to single-phase aggregates such as quartzite and limestone. We present the first results for a polyphase aggregate, peridotite, consisting of 70% olivine and 30% enstatite. The problem is approached with a viscoplastic self-consistent theory satisfying stress equilibrium and strain compatibility for the average polycrystal and taking account of anisotropic neighbor interactions. It is assumed that olivine deforms by (010)[100], (001)[100], and (010)[001] slip and enstatite deforms by (100)[001] slip. Simulated textures for olivine and enstatite in peridotite resemble simulated textures in the pure phases, indicating that for this system and for these volume fractions there is little influence of the different phases upon each other. In our model the harder mineral enstatite deforms at a slower rate than olivine. Interaction between neighboring grains appears to be minimal, which may be due to model assumptions. Predicted pole figures with olivine (010) axes and enstatite (100) axes aligning with the direction of shortening are in good agreement with preferred orientations in naturally and experimentally deformed peridotites.

Electron microscopy in mineralogy

Section 1 Introduction.- Section 2 Contrast.- 2.1 Fundamentals of Electron Microscopy.- 2.2 Interpretation of Electron Diffraction Patterns.- 2.3 Contrast Effects at Planar Interfaces.- 2.4 Computer Simulation of Dislocation Images in Quartz.- 2.5 The Direct Imaging of Crystal Structures.- 2.6 A Comparison of Bright Field and Dark Field Imaging of Pyrrhotite Structures.- Section 3 Experimental Techniques.- Section 4 Exsolution.- 4.1 Exsolution in Silicates.- 4.2 Coarsening in a Spinodally Decomposing System: TiO2-SnO2.- 4.3 Magnetite Lamellae in Reduced Hematites.- 4.4 Precipitation in the Ilmenite-hematite System.- 4.5 Pigeonite Exsolution from Augite.- 4.6 The Transformation of Pigeonite to Orthopyroxene.- 4.7 On the Detailed Structure of Ledges in an Augite-enstatite Interface.- 4.8 The Phase Distributions in Some Exsolved Amphiboles.- 4.9 Physical Aspects of Exsolution in Natural Alkali Feldspars.- 4.10 Analytical Electron Microscopy of Exsolution Lamellae in Plagioclase Feldspars.- 4.11 Exsolution in Metamorphic Bytownite.- Section 5 Polymorphic Phase Transitions.- 5.1 Polymorphic Phase Transitions in Minerals.- 5.2 Direct Observation of Iron Vacancies in Polytypes of Pyrrhotite.- 5.3 Rutile: Planar Defects and Derived Structures.- 5.4 High-resolution Electron Microscopy of Unit Cell Twinning in Enstatite.- 5.5 Polytypism in Wollastonite.- 5.6 High-resolution Electron Microscopy of Labradorite Feldspar.- 5.7 Origin of the (c) Domains of Anorthite.- 5.8 On Polymorphism of BaAl2Si2O8.- 5.9 The Submicroscopic Structure of Wenkite.- Section 6 Deformation Defects.- 6.1 Deformation Structures in Minerals.- 6.2 Work Hardening and Creep Deformation of Corundum Single Crystals.- 6.3 Dislocation Structures in Synthetic Quartz.- 6.4 The Microstructure of Some Naturally Deformed Quartzites.- 6.5 Defects in Deformed Calcite and Carbonate Rocks.- 6.6 Plasticity of Olivine in Peridotites.- 6.7 The Role of Crystal Defects in the Shear-induced Transformation of Orthoenstatite to Clinoenstatite.- Section 7 Special Techniques and Applications.- 7.1 Amorphous Materials.- 7.2 Signals Excited by the Scanning Beam.- 7.3 Analytical Electron Microscopy of Minerals.- 7.4 X-ray Microanalysis Using a Scanning Electron Microscope.- 7.5 Quantitative X-ray Microanalysis of Thin Foils.- 7.6 Particle Track Studies.- 7.7 Stony Meteorites.- 7.8 Microcracks in Crystalline Rocks.

Mollusc shell microstructures and crystallographic textures

Abstract X-ray diffraction is used to characterise textures of the aragonite layers of shells from monoplacophoras, bivalves, cephalopods and gastropods. Textures vary in strength, pattern and through the thickness of the shells. The texture patterns exhibited in the studied taxa, which can be quantitatively described by a limited number of parameters, are compared with the microstructure types observed with scanning electron microscopy. Whereas for simple crystallite arrangements, such as nacres, there is a good correspondence between texture and microstructure, this is often not the case in more complex microstructures such as in crossed lamellar layers. Morphologically similar microstructures may have different crystallographic textures, and the same textures may be found in microstructures with different morphology. These two kinds of measurements are shown to be complementary since they provide non-redundant information for many taxa, which suggests that they may be valuable phylogenetic indicators.

Are pseudotachylites products of fracture or fusion

The microstructure of pseudotachylite veins, as analyzed with the transmission electron microscope, displays many features of intense brittle deformation and resembles more closely shock-deformed specimens than a rock first melted and then quenched to a glass and devitrified upon cooling. Only very minor glass or devitrification textures were observed, but there is ample evidence for recrystallization of highly strained regions. Pseudotachylites appear to form by a dominantly brittle mechanism at moderate to deep crustal levels and may be related to earthquakes.

Deformation of (Mg,Fe)SiO3 Post-Perovskite and D'' Anisotropy

Polycrystalline (Mg0.9,Fe0.1)SiO3 post-perovskite was plastically deformed in the diamond anvil cell between 145 and 157 gigapascals. The lattice-preferred orientations obtained in the sample suggest that slip on planes near (100) and (110) dominate plastic deformation under these conditions. Assuming similar behavior at lower mantle conditions, we simulated plastic strains and the contribution of post-perovskite to anisotropy in the D″ region at the Earth core-mantle boundary using numerical convection and viscoplastic polycrystal plasticity models. We find a significant depth dependence of the anisotropy that only develops near and beyond the turning point of a downwelling slab. Our calculated anisotropies are strongly dependent on the choice of elastic moduli and remain hard to reconcile with seismic observations.