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Rare earth elements (REEs, ‘lanthanides’) constitute a vital commodity for technological applications. Although these elements occur at trace levels in many minerals, they can comprise major constituents of low abundance phosphate, carbonate, silicate and oxide minerals, some of which form during granite weathering. REE-phosphate phases can be a source of phosphorus for essential biomolecules and certain REEs are required by some bacterial enzymes involved in the oxidation of methanol, an important compound in the global biogeochemical carbon cycle. The mechanisms that promote the dissolution of lanthanide phosphate minerals are largely unknown, but probably vary with the lanthanide phosphate mineralogy of weathered rock and soil. Here, we studied weathering of five I-type, three S-type and one A-type granite to determine the extent of weathering of primary REE- and/or P-bearing minerals apatite, allanite and monazite, and the formation of secondary REE/P-bearing minerals. We found evidence for greater mobilisation of REE and P in weathered I-type and A-type granites than in S-types, reflecting the higher solubility of apatite and allanite relative to monazite. Although monazite persisted in highly weathered S-type granites, some alteration was detected. Secondary REE/P-bearing minerals were not detected in two S-type profiles, while spherical secondary REE/P-bearing mineral aggregates were abundant throughout the third S-type profile. Secondary euhedral REE/P-bearing crystals were abundant even in the slightly weathered I-type and A-type granite material, yet they were not detected in the highly weathered material, indicating that these minerals had dissolved. Our findings indicate that mineralogy constrains substantially, but does not control completely, lanthanide availability as a function of degree of weathering. These results have implications for predicting REE and phosphate bioavailability in soils derived from granitic rock types and suggest that highly weathered I-type granites may provide inocula for bioleaching experiments.
Ceramics are strong but brittle. According to the classical theories, ceramics are brittle mainly because dislocations are suppressed by cracks. Here, the authors report the combined elastic and plastic deformation measurements of nanoceramics, in which dislocation-mediated stiff and ductile behaviors were detected at room temperature. In the synchrotron-based deformation experiments, a marked slope change is observed in the stress–strain relationship of MgAl2O4 nanoceramics at high pressures, indicating that a deformation mechanism shift occurs in the compression and that the nanoceramics sample is elastically stiffer than its bulk counterpart. The bulk-sized MgAl2O4 shows no texturing at pressures up to 37 GPa, which is compatible with the brittle behaviors of ceramics. Surprisingly, substantial texturing is seen in nanoceramic MgAl2O4 at pressures above 4 GPa. The observed stiffening and texturing indicate that dislocation-mediated mechanisms, usually suppressed in bulk-sized ceramics at low temperature, become operative in nanoceramics. This makes nanoceramics stiff and ductile.
In patients with subarachnoid haemorrhage (SAH) and a negative finding on CT angiography (CTA), further imaging with digital subtraction angiography (DSA) is commonly performed to identify the source of bleeding. The purpose of this study was to investigate whether negative findings on CTA can reliably exclude aneurysms in patients with acute SAH.
This retrospective study identified all DSAs performed between August 2010 and July 2014 within our institution. CT angiography was performed with a 64-section multidetector row CT scanner. Only DSAs from patients with confirmed SAH and a negative CTA result were included in the final analyses. A fellowship-trained neuroradiologist reviewed the imaging results.
Of the 857 DSAs, 50 (5.83%) were performed in 35 patients with CTA-negative SAH. Of the 35 patients, three (8.57%) had positive findings on the DSA. In one patient, suspicious dissection of the extra- and intra-cranial segment of the right vertebral artery could not be confirmed even in retrospect. In the second patient, the suspicious finding of tiny protuberance from the left paraclinoid internal carotid artery (ICA) on DSA did not change on follow-up and did not change patient’s management. The third patient had a posterior inferior cerebellar artery aneurysm, which was not seen on the initial CTA owing to the incomplete coverage of the head on the CTA.
In patients with SAH, negative findings on a technically sound CTA are reliable in ruling out aneurysms in any pattern of SAH or no blood on CT. Our observations need to be confirmed with larger prospective studies.
Recent ex situ observations of crystallization in both natural and synthetic systems indicate that the classical models of nucleation and growth are inaccurate. However, in situ observations that can provide direct evidence for alternative models have been lacking due to the limited temporal and spatial resolution of experimental techniques that can observe dynamic processes in a bulk solution. Here we report results from liquid cell transmission electron microscopy studies of nucleation and growth of Au, CaCO3, and iron oxide nanoparticles. We show how these in situ data can be used to obtain direct evidence for the mechanisms underlying nanoparticle crystallization as well as dynamic information that provide constraints on important energetic parameters not available through ex situ methods.
Chromium-doped (0.5-10 % Cr:Ti molar ratio) nanocrystalline titania (5–6 nm) prepared via sol-gel method was examined by synchrotron-based wide angle x-ray scattering (WAXS) for crystal structure determination. Atomic pair-distribution functions (PDF) for both raw and heat-treated samples were obtained by Fourier transforms of the WAXS data. The PDF data were fitted using structural models of nanocrystalline titania that considered phase compositions, lattice parameters, atomic positions and thermal factors. The unit cell of Cr-doped nanocrystalline titania expanded 1-2 % with respect to bulk titania as a consequence of the substitution of Ti by Cr and the generation of oxygen vacancies. We observed a lattice contraction after heat-treatment that may be caused by the redistribution of Cr atoms to nanoparticle surfaces during phase transformation and particle coarsening.
We interpret recent spectral data of Mars collected by the Mars Exploration Rovers to contain substantial evidence of sulfate minerals and aqueous processes. We present visible/near-infrared (VNIR), mid-IR and Mössbauer spectra of several iron sulfate minerals and two acid mine drainage (AMD) samples collected from the Iron Mountain site and compare these combined data with the recent spectra of Mars. We suggest that the sulfates on Mars are produced via aqueous oxidation of sulfides known to be present on Mars from Martian meteorites. The sulfate-rich rock outcrops observed in Meridiani Planum may have formed in an acidic environment similar to AMD environments on Earth. Because microorganisms are typically involved in the oxidation of sulfides to sulfates in terrestrial AMD sites, sulfate-rich rock outcrops on Mars may be a good location to search for evidence of life on that planet. Whether or not life evolved on Mars, following the trail of sulfate minerals is likely to lead to aqueous processes and chemical weathering. Our results imply that sulfate minerals formed in Martian soils via chemical weathering, perhaps over very long time periods, and that sulfate minerals precipitated following aqueous oxidation of sulfides to form the outcrop rocks at Meridiani Planum.
The kinetics of phase transformation of nanocrystalline anatase samples was studied using x-ray diffraction at temperatures ranging from 600 to 1150 °C. Kinetic data were analyzed with an interface nucleation model and a newly proposed kinetic model for combined interface and surface nucleation. Results revealed that the activation energy of nucleation is size dependent. In anatase samples with denser particle packing, rutile nucleates primarily at interfaces between contacting anatase particles. In anatase samples with less dense particle packing, rutile nucleates at both interfaces and free surfaces of anatase particles. The predominant nucleation mode may change from interface nucleation at low temperatures to surface nucleation at intermediate temperatures and to bulk nucleation at very high temperatures. Alumina particles dispersed among the anatase particles can effectively reduce the probability of interface nucleation at all temperatures.
Initial thermodynamic analysis predicts surface tension of fine solid particles decreases with decrease in particle size. Free energy of fine solid particles increases with decrease in particle size. In the nanocrystalline TiO2 system, the surface tensions and the surface energies of both anatase and rutile were estimated by modeling available thermochemical data and kinetic data. The particle size versus temperature phase diagram of the nanocrystalline TiO2 system was calculated, which reveals 1–8 nm anatase is more stable than rutile of the same size at certain temperatures.
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