ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉Carbon is found in nature in a huge variety of allotropic forms and recent research in materials science has encouraged the development of technological materials based on nanocarbon. Carbon atoms with 〈span〉sp〈/span〉〈sup〉2〈/sup〉 or 〈span〉sp〈/span〉〈sup〉3〈/sup〉 hybridization can be thought of as building blocks. Following a bottom-up approach, we show how graphene and diamond molecules are built up and how their properties vary with size, reaching an upper limit with bulk graphite and diamond. Carbon atoms with 〈span〉sp〈/span〉〈sup〉2〈/sup〉 hybridization give rise to an impressive number of different materials, such as carbon nanotubes, graphene nanoribbons, porous carbon and fullerene. As in any crystalline phase, the crystal structures of natural carbon allotropes (i.e. graphite and diamond) contain various types of imperfections. These so-called lattice defects are classified by their dimensions into 0D (point), 1D (line), 2D (planar) and 3D (volume) defects. Lattice defects control the physical properties of crystals and are often a fingerprint of the geological environment in which they formed and were modified. Direct observations of lattice defects are commonly accomplished by transmission electron microscopy. We present and discuss the ideal and real structures of carbon allotropes, the energetics of lattice defects and their significance in understanding geological processes and conditions.〈/span〉

Description: The position of the spinel-post-spinel phase transition in Fe3O4 has been determined in pressure-temperature space by in situ measurements using a multi-anvil press combined with white synchrotron radiation. Pressure measurement using the equation of state for MgO permitted pressure changes to be monitored at high temperature. The phase boundary was determined by the first appearance of diffraction peaks of the high-pressure polymorph (h-Fe3O4) during pressure increase and the disappearance of these peaks on pressure decrease along several isotherms. We intersected the phase boundary over the temperature interval of 700-1400 {degrees}C. The boundary is linear and nearly isobaric, with a slightly positive slope. Post-experiment investigation by TEM confirms that the reverse reaction from h-Fe3O4 to magnetite during decompression leads to the formation of microtwins on the (311) plane in the newly formed magnetite. Observations made during the phase transition suggest that the transition has a pseudomartensitic character, explaining in part why magnetite persists at conditions well within the stability field of h-Fe3O4, even at high temperatures. This study emphasizes the utility of studying phase transitions in situ at simultaneously high temperatures and pressures since the reaction kinetics may not be favorable at room temperature.

Description: Hibonite single crystals were synthesized using two different techniques: hot-pressing of polycrystalline hibonite by means of piston-cylinder apparatus and solid state reaction using citrate-based sol-gel precursors. Chemical analyses, UV/Vis spectroscopy, and single-crystal X-ray characterization were performed on four sets of Ti-Mg-bearing hibonites to investigate the substitution mechanism of Ti 3+ , Ti 4+ , and Mg 2+ , relevant for hibonites found in calcium-aluminum-rich inclusions in meteorites. The unit-cell volume of hibonite depends linearly on the concentration of Ti and Mg: V = 8.21(3)·(Ti tot + Mg) apfu + 586.06(1). Structural refinements, carried out in the space group P 6 3 / mmc , demonstrate that Ti occupies two sites: M2, a trigonal bipyramid, and M4, a distorted octahedron occurring in face-sharing pairs. The Ti occupancy factor was refined at both sites. Due to the repulsion of neighboring Ti cations the bond distance M4-O3 increases with increasing Ti content and the cations are displaced from the central position of the polyhedron. The displacement factors ( U 33 ) M2 and the site occupancy factor for Ti in the M2 site positively correlate for the samples, which have more than 0.3–0.4 Ti apfu , while ( U 33 ) M2 remains that of pure hibonite for small Ti occupancies at the M2 site. This plateau of displacement factor reflects the local strain fields around the substituted Ti atoms and its magnitude indicates that these strain fields begin to overlap at a Ti-Ti separation of about 1–2 unit cells. For a sample synthesized at low oxygen fugacity we detected Ti 3+ by means of UV/Vis absorption spectroscopy. The presence of Ti 3+ has a clear effect on the M4-M4 distance, which deviates from the linear trend described by samples containing mainly Ti 4+ . The average bond length M3-O depends linearly on the Mg content of these synthetic hibonites clearly indicating that Mg occupies this site.

Description: Extensive negative aeromagnetic anomalies in the Modum area, south Norway, derive from rocks containing ilmenite with hematite exsolution, or hematite with ilmenite exsolution, carrying strong/stable reversed remanence. Here we describe a 2.5 cm thick high-temperature metamorphic vein of exsolved titanohematite. Reflected-light and EMP analyses show it contains three types of exsolution: spinel plates on (001); rutile blade satellites on spinel oriented at angles of ~60–90° to titanohematite (001); and lamellae 0.1–0.3 μm thick too fine for EMP analyses, also parallel to (001). Powder XRD gave a = 5.0393 Å, c = 13.7687 Å, V = 302.81 Å 3 for titanohematite (Ilm9), and unrefined reflections of rutile and geikielite. Overlap EMP analyses showed enrichment in MgO, TiO 2 , and lack of Al 2 O 3 , indicating a mixture of titanohematite and geikielite. Non-overlap analyses showed the titanohematite is 6%Fe 2+ TiO 3 , 2%MgTiO 3 , 92% Fe 2 O 3 , generally confirmed by TEM-EDX analyses that also showed the geikielite is 30%Fe 2+ TiO 3 , 70%MgTiO 3 . Orientation and interface relationships between exsolutions and host titanohematite were characterized with TEM, using conventional and high-resolution imaging complemented by selected-area electron diffraction. Spinel shares (111) with (001) of titanohematite and geikielite (001) the same. The epitactic relationship between rutile and titanohematite, previously not well constrained, was estimated from reflected-light and TEM images and lattice-fit studies. The a 1 axis of rutile is parallel to a 1 of hematite and c of rutile is normal to a 2 of hematite, all in the hematite basal plane, which, however is not a phase interface. The rutile appears to occur in blades within prism planes in titanohematite located ~69° from a axes of hematite, with long axes of the blades oriented in a minimum strain direction within the planes at ~63° from the (001) basal plane. Spinel and rutile, analyzed by EMP, exsolved first. Spinel gave 96%MgAl 2 O 4 , 3%FeFe 2 O 4 , Mg/total R 2+ = 0.98. Magnesian/aluminous spinel lacking Ti exsolved from titanohematite in coupled exsolution with ferrian rutile, where combined components were dissolved as corundum/geikielite components in high- T aluminous magnesian titanohematite. Early exsolution lowered geikielite, and eliminated the corundum component. Later fine exsolution of ferroan geikielite moved the titanohematite closer to Fe 2 O 3 . Mg 2+ has no magnetic moment, but breaks up linkages between Fe atoms, lowers Néel T s, and produces unusual low- T properties. This titanohematite has Néel T , 873 K (600 °C). Geikielite at 70%MgTiO 3 , is far beyond its theoretical nearest-neighbor percolation threshold at 30.3%MgTiO 3 . However, the sample shows a negative magnetic exchange bias below 25 K and low- T remanence lost above ~40 K. Such properties are reported in samples containing thin ilmenite lamellae in titanohematite, in theory with odd numbers of Fe layers, where exchange bias is linked to lamellar magnetism at the phase interfaces, when the ilmenite becomes a high-anisotropy magnet in a magnetically softer host. Potential explanations for the behavior of ferroan geikileite are discussed.