With in situ nanocompression experiments in a transmission electron microscope, we investigated plastic deformation in natural quartz crystals and observed both dislocation plasticity as well as mechanical twinning. Through this experimental method, we are able to provide direct evidence of Dauphiné twin nucleation and could measure the intrinsic twinning stress. The twinning phenomena appear to include a memory effect, where the same twin can reappear upon successive loading and unloading events. The data provide insight into this twin generation mechanism and can be used as a benchmark for the use of twins in quartz for paleopiezometry. Together, the observation of room-temperature dislocation plasticity and reversible twinning adds new insight into the extensive field of quartz plasticity and demonstrates the usefulness of small-scale testing techniques for mineral physics.

We became aware of uncertainties about lattice parameters for osmium in Table 1 and B.K. Godwal (with the Department of Earth and Planetary Science at UC Berkeley) has reanalyzed the radial diamond anvil cell diffraction data, measured in situ at high pressure and compressive stress, with the Moment Pole Stress model in an advanced MAUD Rietveld technique (Wenk et al., 2014), taking into account lattice distortion under stress. [Table presented] The new results compare much better with data from hydrostatic experiments (Godwal et al., 2012). Since the emphasis of the paper was on mechanical twinning, we did not pay much attention to lattice distortion. Note that standard deviations are based on the Rietveld fit.

Transition metals Ti, Zr, and Hf have a hexagonal close-packed structure (α) at ambient conditions, but undergo phase transformations with increasing temperature and pressure. Of particular significance is the high-pressure hexagonal ω phase which is brittle compared to the α phase. There has been a long debate about transformation mechanisms and orientation relations between the two crystal structures. Here we present the first high pressure experiments with in situ synchrotron x-ray diffraction texture studies on polycrystalline aggregates. We follow crystal orientation changes in Zr, confirming the original suggestion by Silcock for an α→ω martensitic transition for Ti, with (0001)(α)||(1120)(ω), and a remarkable orientation memory when ω reverts back to α.

Slates are low-grade metamorphic argillaceous rocks which have developed an intense tectonic fabric, referred to as slaty cleavage. Due to their unique properties they have long been used as building materials, especially tiles for roofs and floors. The fabric, characterized by strong alignment of platy phyllosilicate crystals, produces extreme anisotropy that has been of great interest in structural geology: What different fabric types are observed? How do they form? Here we review crystal preferred orientation (CPO) and add new information to establish a comprehensive base for future studies. Slates are composed of fine-grained phyllosilicates (dominantly white mica and chlorite, sometimes chloritoid), quartz and accessories. Texture analyses of these polyphase rocks with high energy synchrotron X-ray diffraction and high resolution EBSD reveal strong alignment of mica and chlorite, in some cases exceeding any previously reported texture strength, including metals. Interestingly, the CPO of quartz, with highly flattened grains (shape preferred orientation or SPO), is practically random. Recrystallization microstructures observed by scanning electron microscopy suggest that the phyllosilicate preferred orientation is largely caused by crystal growth under stress, and that quartz was subject to pressure solution and reprecipitation, producing an anisotropic shape. While phyllosilicates preferred orientation in slates is much higher than reports for shale, schists and gneiss, elastic seismic anisotropy for slates is similar to shales where a significant component is due to aligned porosity.

Felsic and mafic gneisses constitute large proportions of the upper and lower continental crust. Gneisses often display high anisotropy of elastic properties associated with preferred orientations of sheet silicates. Here we study the elastic anisotropy of a sample of Tambo gneiss from Promontogno in the Central Alps. We apply optical microscopy, time-of-flight neutron diffraction, neutron and X-ray tomography to quantify mineral composition and microstructures and use them to construct self-consistent models of elastic properties. They are compared to results of ultrasonic measurements on a cube sample in a multi-anvil apparatus and on a spherical sample in an apparatus that can measure velocities in multiple directions. Both methods provide similar results. It is shown that models of microstructure-derived elastic properties provide a good match with ultrasonic experiment results at pressures above 100 MPa. At a pressure of 0.1 MPa the correspondence between the model and the experiment is worse. This may be caused by an oversimplification of the model with respect to microfractures or uncertainties in the experimental determination of S-wave velocities and elastic tensor inversion. The study provides a basis to determine anisotropic elastic properties of rocks either by ultrasonic experiments or quantitative models based on microstructures. This information can then be used for interpretation of seismic data of the crust.

The microstructures, mineralogy and chemistry of representative samples collected from the cores of the San Andreas Fault drill hole (SAFOD) and the Taiwan Chelungpu-Fault Drilling project (TCDP) have been studied using optical microscopy, TEM, SEM, XRD and XRF analyses. SAFOD samples provide a transect across undeformed host rock, the fault damage zone and currently active deforming zones of the San Andreas Fault. TCDP samples are retrieved from the principal slip zone (PSZ) and from the surrounding damage zone of the Chelungpu Fault. Substantial differences exist in the clay mineralogy of SAFOD and TCDP fault gouge samples. Amorphous material has been observed in SAFOD as well as TCDP samples. In line with previous publications, we propose that melt, observed in TCDP black gouge samples, was produced by seismic slip (melt origin) whereas amorphous material in SAFOD samples was formed by comminution of grains (crush origin) rather than by melting. Dauphiné twins in quartz grains of SAFOD and TCDP samples may indicate high seismic stress. The differences in the crystallographic preferred orientation of calcite between SAFOD and TCDP samples are significant. Microstructures resulting from dissolution-precipitation processes were observed in both faults but are more frequently found in SAFOD samples than in TCDP fault rocks. As already described for many other fault zones clay-gouge fabrics are quite weak in SAFOD and TCDP samples. Clay-clast aggregates (CCAs), proposed to indicate frictional heating and thermal pressurization, occur in material taken from the PSZ of the Chelungpu Fault, as well as within and outside of the SAFOD deforming zones, indicating that these microstructures were formed over a wide range of slip rates. © 2014 Elsevier Ltd.