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© 2016 The lower mantle is estimated to be composed of mostly bridgmanite and a smaller percentage of ferropericlase, yet very little information exists for two-phase deformation of these minerals. To better understand the rheology and active deformation mechanisms of these lower mantle minerals, especially dislocation slip and the development of crystallographic preferred orientation (CPO), we deformed mineral analogs neighborite (NaMgF3, iso-structural with bridgmanite) and halite (NaCl, iso-structural with ferropericlase) together in the deformation-DIA at the Advanced Photon Source up to 51% axial shortening. Development of CPO was recorded in situ with X-ray diffraction, and information on microstructural evolution was collected using X-ray microtomography. Results show that when present in as little as 15% volume, the weak phase (NaCl) controls the deformation. Compared to single phase NaMgF3samples, samples with just 15% volume NaCl show a reduction of CPO in NaMgF3and weakening of the aggregate. Microtomography shows both NaMgF3and NaCl form highly interconnected networks of grains. Polycrystal plasticity simulations were carried out to gain insight into slip activity, CPO evolution, and strain and stress partitioning between phases for different synthetic two-phase microstructures. The results suggest that ferropericlase may control deformation in the lower mantle and reduce CPO in bridgmanite, which implies a less viscous lower mantle and helps to explain why the lower mantle is fairly isotropic.

Magnesiowustite, (Mg0.08Fe0.88)O, and wustite, Fe0.94O, were compressed to ~36 GPa at ambient temperature in the diamond anvil cell (DAC) at the Advanced Light Source. X-ray diffraction patterns were taken in situ in radial geometry in order to study the evolution of crystallographic preferred orientation through the cubic-to-rhombohedral phase transition. Under uniaxial stress in the DAC, {100}c planes aligned perpendicular to the compression direction. The {100}c in cubic became { $$\left\{ {10\bar 14} \right\}$$ }r in rhombohedral and remained aligned perpendicular to the compression direction. However, the {101}c and {111}c planes in the cubic phase split into { $${10{\bar{1}}4}$$ }r and { $${11{\bar{2}}0}$$ }r, and (0001)r and { $${10{\bar{1}}1}$$ }r, respectively, in the rhombohedral phase. The { $${11{\bar{2}}0}$$ }r planes preferentially aligned perpendicular to the compression direction while { $${10{\bar{1}}4}$$ }r oriented at a low angle to the compression direction. Similarly, { $${10{\bar{1}}1}$$ }r showed a slight preference to align more closely perpendicular to the compression direction than (0001)r. This variant selection may occur because the 〈 $${10{\bar{1}}4}$$ 〉r and [0001]r directions are the softer of the two sets of directions. The rhombohedral texture distortion may also be due to subsequent deformation. Indeed, polycrystal plasticity simulations indicate that for preferred { $${10{\bar{1}}4}$$ }〈 $${1{\bar{2}}10}$$ 〉r and { $${11{\bar{2}}0}$$ }〈 $${{\bar{1}}101}$$ 〉r slip and slightly less active { $${10{\bar{1}}1}$$ }〈 $${{\bar{1}}2{\bar{1}}0}$$ 〉r slip, the observed texture pattern can be obtained.

Diamond anvil cells (DAC) in radial synchrotron X-ray diffraction geometry were used to investigate texture development and identify deformation mechanisms in zinc and osmium at the Advanced Photon Source (APS) and the Advanced Light Source (ALS), respectively. Further experiments on cadmium and hafnium wires were carried out in the Deformation-DIA (D-DIA) multi-anvil press at APS to study the simultaneous effects of pressure, temperature and strain. At room temperature and increasing pressure the c -axis aligns near the compression direction in all hexagonal metals, but with considerable differences. Texture in zinc evolves gradually between 10 and 15 GPa and strengthens as pressure is increased to 25 GPa. In osmium, texture development starts very early (4 GPa). At ambient temperature cadmium and hafnium develop a similar textures as zinc and osmium, respectively. Texture in cadmium evolves gradually with axial shortening to 34%, whereas in hafnium texturing develops immediately after small strains. When hafnium is simultaneously heated to 700 K and deformed in compression, a texture develops with compression axes near ( 2 1 ¯ 1 ¯ 0 ) . Simulations from a visco-plastic self-consistent (VPSC) polycrystal plasticity model suggest that the gradual texture evolution observed in zinc and cadmium is controlled primarily by { 0 0 0 1 } 〈 2 1 ¯ 1 ¯ 0 〉 basal slip and later accompanied by { 1 0 1 ¯ 2 } 〈 1 ¯ 0 1 1 〉 tensile twinning when the c / a ratio is below 3 ≈ 1.732 . Conversely, early texture development in osmium and hafnium at room temperature is contributed mainly by { 1 0 1 ¯ 2 } 〈 1 ¯ 0 1 1 〉 tensile twinning. However, the ( 2 1 ¯ 1 ¯ 0 ) texture in hafnium at high temperature is attributed to basal and prismatic slip.

© 2014 by Walter de Gruyter Berlin/Boston. Plagioclase feldspars comprise a large portion of the Earth's crust and are very anisotropic, making accurate knowledge of their elastic properties important for understanding the crust's anisotropic seismic signature. However, except for albite, existing elastic constants for plagioclase feldspars are derived from measurements that cannot resolve the triclinic symmetry. We calculate elastic constants for plagioclase end-members albite NaAlSi3O8and anorthite CaAl2Si2O8and intermediate andesine/ labradorite NaCaAl3Si5O16using density functional theory to compare with and improve existing elastic constants and to study trends in elasticity with changing composition. We obtain elastic constants similar to measured elastic constants and find that anisotropy decreases with anorthite content.

Author(s): Kanitpanyacharoen, W; Merkel, S; Miyagi, L; Kaercher, P; Tome, CN; Wang, Y; Wenk, HR | Abstract: 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.

© International Centre for Diffraction Data 2014. Synchrotron X-ray diffraction images are increasingly used to characterize crystallographic preferred orientation distributions (texture) of fine-grained polyphase materials. Diffraction images can be analyzed quantitatively with the Rietveld method as implemented in the software package Materials Analysis Using Diffraction. Here we describe the analysis procedure for diffraction images collected with high energy X-rays for a complex, multiphase shale, and for those collected in situ in diamond anvil cells at high pressure and anisotropic stress.

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.

Slippery When Squeezed The behavior of seismic waves as they pass through Earth's interior depends on the physical properties of major mineral phases at depth. If such minerals are anisotropic—that is, they influence seismic waves preferentially depending on crystallographic orientation—interpreting the structure of a region becomes more challenging. In the lowermost mantle, near the boundary with the outer core, deformation of MgSiO 3 post-perovskite may affect anisotropy. Miyagi et al. (p. 1639 ) solved previous experimental limitations to show that, when squeezed at high pressures, MgSiO 3 post-perovskite weakens and breaks along its (001) lattice plane. When modeled, this deformation pattern produces anisotropic structures that are consistent with seismic data collected from this region.

To extend the range of high-temperature, high-pressure studies within the diamond anvil cell, a Liermann-type diamond anvil cell with radial diffraction geometry (rDAC) was redesigned and developed for synchrotron X-ray diffraction experiments at beamline 12.2.2 of the Advanced Light Source. The rDAC, equipped with graphite heating arrays, allows simultaneous resistive and laser heating while the material is subjected to high pressure. The goals are both to extend the temperature range of external (resistive) heating and to produce environments with lower temperature gradients in a simultaneously resistive- and laser-heated rDAC. Three different geomaterials were used as pilot samples to calibrate and optimize conditions for combined resistive and laser heating. For example, in Run#1, FeO was loaded in a boron-mica gasket and compressed to 11 GPa then gradually resistively heated to 1007 K (1073 K at the diamond side). The laser heating was further applied to FeO to raise temperature to 2273 K. In Run#2, Fe-Ni alloy was compressed to 18 GPa and resistively heated to 1785 K (1973 K at the diamond side). The combined resistive and laser heating was successfully performed again on (Mg0.9Fe0.1)O in Run#3. In this instance, the sample was loaded in a boron-kapton gasket, compressed to 29 GPa, resistive-heated up to 1007 K (1073 K at the diamond side), and further simultaneously laser-heated to achieve a temperature in excess of 2273 K at the sample position. Diffraction patterns obtained from the experiments were deconvoluted using the Rietveld method and quantified for lattice preferred orientation of each material under extreme conditions and during phase transformation.