Waruntorn Kanitpanyacharoen

Slip Systems in MgSiO3 Post-Perovskite: Implications for D′′ Anisotropy

Slippery When Squeezed The behavior of seismic waves as they pass through Earths 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 MgSiO3 post-perovskite may affect anisotropy. Miyagi et al. (p. 1639) solved previous experimental limitations to show that, when squeezed at high pressures, MgSiO3 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. The major mineral phase in the lower mantle deforms preferentially along one lattice plane. Understanding deformation of mineral phases in the lowermost mantle is important for interpreting seismic anisotropy in Earth’s interior. Recently, there has been considerable controversy regarding deformation-induced slip in MgSiO3 post-perovskite. Here, we observe that (001) lattice planes are oriented at high angles to the compression direction immediately after transformation and before deformation. Upon compression from 148 gigapascals (GPa) to 185 GPa, this preferred orientation more than doubles in strength, implying slip on (001) lattice planes. This contrasts with a previous experiment that recorded preferred orientation likely generated during the phase transformation rather than deformation. If we use our results to model deformation and anisotropy development in the D′′ region of the lower mantle, shear-wave splitting (characterized by fast horizontally polarized shear waves) is consistent with seismic observations.

Texture of Nanocrystalline Nickel: Probing the Lower Size Limit of Dislocation Activity

Strength Under Pressure Above a lower cutoff value, shrinking the grain size of a metal tends to strengthen it because the overall increase in grain boundaries limits the activity of the dislocations as the material undergoes plastic deformation. Chen et al. (p. 1448) explore the question of whether this restriction in dislocation activity occurs when a metal is subjected to high pressures. Foils of nickel made from particles of different sizes were subjected to high pressures inside a diamond anvil cell. An increase in pressure extended dislocation activity to smaller grain sizes, indicating that pressure compensates for the inhibition of dislocation activity in small volumes. A metal subjected to high pressures retains the activity of dislocations, even for very small grain sizes. The size of nanocrystals provides a limitation on dislocation activity and associated stress-induced deformation. Dislocation-mediated plastic deformation is expected to become inactive below a critical particle size, which has been proposed to be between 10 and 30 nanometers according to computer simulations and transmission electron microscopy analysis. However, deformation experiments at high pressure on polycrystalline nickel suggest that dislocation activity is still operative in 3-nanometer crystals. Substantial texturing is observed at pressures above 3.0 gigapascals for 500-nanometer nickel and at greater than 11.0 gigapascals for 20-nanometer nickel. Surprisingly, texturing is also seen in 3-nanometer nickel when compressed above 18.5 gigapascals. The observations of pressure-promoted texturing indicate that under high external pressures, dislocation activity can be extended down to a few-nanometers-length scale.

Rietveld texture analysis from synchrotron diffraction images. II. Complex multiphase materials and diamond anvil cell experiments

© 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.

MINERAL PREFERRED ORIENTATION AND MICROSTRUCTURE IN THE POSIDONIA SHALE IN RELATION TO DIFFERENT DEGREES OF THERMAL MATURITY

The thermal maturity of samples of the Posidonia Shale collected from the Hils Syncline, northern Germany, varies significantly as a function of location indicating variations in local history. Synchrotron X-ray diffraction was used to document the composition and the preferred orientation of four samples of the Posidonia Shale with different degrees of maturity (0.68–1.45%, Ro) to determine possible effects on diagenesis and preferred orientation. Overall, the degree of preferred orientation of all clay minerals (illite-smectite, illite-mica, and kaolinite) and in all samples is similar, with (001) pole figure maxima ranging from 3.7 to 6.3 multiples of a random distribution (m.r.d.). Calcite displays weak preferred orientation, with c axes perpendicular to the bedding plane (1.1–1.3 m.r.d.). Other constituent phases such as quartz, feldspars, and pyrite have a random orientation distribution. The difference in thermal history, which causes significant changes in the maturity of organic matter, influenced the preferred orientation of clay minerals only marginally as most of the alignment seems to have evolved early in their history. Synchrotron X-ray microtomography was used to characterize the three-dimensional microstructure of a high-maturity sample. Low-density features, including porosity, fractures, and kerogen, were observed to be elongated and aligned roughly parallel to the bedding plane. The volume of low-density features was estimated to be ~7 vol.%, consistent with previous petrophysical measurements of porosity of 8–10 vol.%. Transmission electron microscopy analysis of samples with different degrees of maturity (0.74%Ro and 1.45%Ro) was used to document microstructures at the nanoscale as well as the presence of kerogen. In the high-maturity sample, pores were less abundant while minerals were more deformed as shown by fractured calcite and by kinked and folded illite. Some of the porosity was aligned with clay platelets.

A comparative study of X-ray tomographic microscopy on shales at different synchrotron facilities: ALS, APS and SLS.

The 3D microstructure of shales is important to assess elastic anisotropic characteristics. In this study, microporosity and mineral components in two shale samples were investigated with X-ray tomographic microscopy at three synchrotron facilities: ALS, APS and SLS, and excellent agreement was observed.

Combined resistive and laser heating technique for in situ radial X-ray diffraction in the diamond anvil cell at high pressure and temperature

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.

Rock physics of fibrous rocks akin to Roman concrete explains uplifts at Campi Flegrei Caldera

Cementing Roman concrete to a caldera Ancient concrete would seem to have little to do with volcano geology. However, Vanorio and Kanitpanyacharoen found similarities between the caprock of the Campi Flegrei caldera near Naples, Italy, and the Roman-era concrete for which the region was known. Both materials require a similar set of chemical reactions to give it the high strength caused by microstructures of intertwining fibrous minerals. The high strength of the natural rock explains the ability of the caldera to withstand periods of high-rate uplift without eruption. The Romans living in the caldera, where the town of Pozzuoli is today, may have been trying to mimic nature to produce this iconic material. Science, this issue p. 617 A natural high-strength caprock, similar to Roman concrete, allows for noneruptive uplift in the Campi Flegrei caldera. Uplifts in the Campi Flegrei caldera reach values unsurpassed anywhere in the world (~2 meters). Despite the marked deformation, the release of strain appears delayed. The rock physics analysis of well cores highlights the presence of two horizons, above and below the seismogenic area, underlying a coupled process. The basement is a calc-silicate rock housing hydrothermal decarbonation reactions, which provide lime-rich fluids. The caprock above the seismogenic area has a pozzolanic composition and a fibril-rich matrix that results from lime-pozzolanic reactions. These findings provide evidence for a natural process reflecting that characterizing the cementitious pastes in modern and Roman concrete. The formation of fibrous minerals by intertwining filaments confers shear and tensile strength to the caprock, contributing to its ductility and increased resistance to fracture.