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Shock-produced melt veins in the Martian shergottite Dar al Gani 670 crosscut large olivine crystals. The upper part of one of these crystals appears to be sheared off and displaced along the shock vein. From the olivine-vein interface small lamellae of ringwoodite grow into the host crystal. The ≤1-3 mm wide and up to 20 mm long lamellae consist of small bands and blocks and are orientated along specific crystallographic orientations. Texture and composition, i.e., more Fe-rich than the host olivine, indicate that lamellae formed via incoherent diffusion-controlled growth. It is suggested that a combination of high particle velocities and shock-induced defects lead to enhanced diffusion rates. In addition, shearing caused grain size reduction allowing rapid Fe-Mg interchange and induced lattice defects serving as nucleation sites for ringwoodite. Crystallographic orientation of ringwoodite lamellae indicates that during shock deformation the [001]{hk0} slip system was activated in olivine. Natural high-pressure phases in Martian meteorite allow to constrain phase transitions taking place in the inaccessible Earth's mantle. High-pressure shear instabilities of olivine at subduction zones in 400-700 km depth are considered being responsible for deep earthquakes. At such p-T-conditions, breakdown of olivine results in formation of ringwoodite filled micro-anticracks which interact with each other finally leading to catastrophic shear failure. Our results strongly suggest that shearing itself contributes to a runaway process of enhanced ringwoodite formation and, thus, reinforces catastrophic material failure that may result in deep earthquakes. & 2013 Elsevier B.V. All rights reserved. trolled, solid-state mechanism during shock compression (e.g., Chen et al., 1996; Sharp and DeCarli, 2006). Furthermore, the homogeneous distributions and random orientation of the crystallites argue against heterogeneous nucleation on grain boundaries and favor homogeneous intracrystalline nucleation of ringwoodite throughout the olivine (e.g., Sharp and DeCarli, 2006). Rare exceptions from this occurrence constitute ringwoodite/ wadsleyite composite grains found in chondrules entrained in shock melt veins (Miyahara et al., 2008) and those found in small $400 mm sized shock melt pockets of the Martian dunite Contents lists available at ScienceDirect

The structure and phase evolution of nanocrystalline Ce1−xLnxO2−x/2−δ (Ln = Yb, Lu, x = 0 − 1) oxides upon heating in H2 was studied for the first time. Up to 950 °C the samples were single-phase, with structure changing smoothly with x from fluorite type (F) to bixbyite type (C). For the Lu-doped samples heated at 1100 °C in the air and H2, phase separation into coexisting F- and C-type structures was observed for ~0.40 < x < ~0.70 and ~0.25 < x < ~0.70, respectively. It was found also that addition of Lu3+ and Yb3+ strongly hinders the crystallite growth of ceria during heat treatment at 800 and 950 °C in both atmospheres. Valency of Ce and Yb in Ce0.1Lu0.9O1.55−δ and Ce0.95Yb0.05O1.975−δ samples heated at 1100 °C was studied by XANES and magnetic measurements. In the former Ce was dominated by Ce4+, with small contribution of Ce3+ after heating in H2. In the latter, Yb existed exclusively as 3+ in both O2 and H2.

Displacive transformations have been widely reported in metals, alloys and ceramics, but rarely reported to be important in the aqueous corrosion of alloys. We report here our analysis of the formation of the hexagonal-ZrO suboxide during the aqueous corrosion of α-Zr alloys and propose this to be a paraequilibrium displacive transformation with the rate controlled by oxygen diffusion. Two orientation relationships were identified between α-Zr and hexagonal-ZrO, ( 0002, with the first one more commonly observed. No specific orientation relationships between either hexagonal-ZrO and monoclinic-ZrO2 or α-Zr and monoclinic-ZrO2 were identified, which suggests that the formation of often-reported bulk oxide texture during aqueous corrosion is not related directly to the texture of the metallic substrate. These results provide a guideline for understanding the mechanisms of crystallographic evolution during oxide growth on commercial zirconium alloys, and also demonstrate the capability of transmission Kikuchi diffraction to investigate orientation relationships in nano-scale materials.