The crystal structure of a terrestrial 6H-SiC moissanite has been reÞ ned in the P6 3 mc S.G. from area detector single crystal X-ray data, down to an R-index on the observed reß ections of 0.0205. The cell parameters reÞ ned over all the collected reß ections are a = 3.0810(2) and c = 15.1248(10) Å. The average Si-C bond lengths are 1.8898 Å, with average bonds along the stacking direction (1.8993 Å) slightly longer than those along the bilayer (1.8862 Å). The interlayer distances, deÞ ned as the distances along [0001] between Si-Si layers, which may occur either in cubic (c) or hexagonal (h) conÞ gurations, are maximal at the c-h interface (2.5270 Å) and minimal at the h-c interface (2.5165 Å), entailing that the h-bilayer is not equidistant from either c-bilayers. All the tetrahedral angles are identical within the experimental error and close to the ideal value of 109.47°, but those at the c-h interface, where a signiÞ cant distortion of 0.15° is recorded. Finally, the anisotropic displacement factors are utterly very small, identical among different atoms within the experimental error, and signiÞ cantly spherical. It thus appears that the 6H-SiC structure is affected by a slight relaxation along the [0001] stacking direction with respect to the ideal cubic structure, and that the relaxation is mainly accomplished at the c-h interface, i.e., at the twin-like boundary, where a bilayer in cubic conÞ guration links a bilayer in antiparallel, hexagonal conÞ guration. As far as we know this is the Þ rst crystal structure reÞ nement of a natural 6H-SiC moissanite. Possible implications on the polytype stability in the light of these results are brieß y discussed.
Diamond BZ270 derives from within a 30 km radius of the small Rio São Luiz, near the town of Juina. The sampling site for diamond JUc4 can be more precisely constrained to 261000 m E and 8708000 m N in WGS84 UTM-projected Zone 21S (Agrosi et al. 2017; Figure DR1). METHODS Micro-Computed X-ray Tomography Micro-Computed X-ray Tomography (Cnudde and Boone, 2013) was carried out using a Scyscan 1172 microtomograph, located at the University of Bari (Italy). A 45-kV X-ray source was used with a current of 218 µA. A total of 1200 absorption radiographs were acquired over a 360 ° rotation with an angular step of 0.3 °. Random movement of the vertical axis and multiple-frame averaging were used to minimize the Poisson noise in the projection images. Beam hardening was reduced by the presence of a 0.5 mm Al-filter between the source and the detector. The nominal spatial resolution for the resulting model was 4.75 µm. The raw data were reconstructed into two-dimensional slice images using the software "NRecon, Skyscan, Belgium". Corrections for the beam-hardening effect and ring artifacts were also applied during the reconstruction process. Micro-CT data were analyzed using the software "CT-analyser, Skyscan, Belgium". Cathodoluminescence Cathodoluminescence images were collected on a Centaurus detector attached to a Hitachi S-3500N scanning electron microscope (SEM; School of Earth Sciences, University of Bristol). Samples were carbon coated prior to imaging. Accelerating voltages were varied between 10 and 20 kV to obtain the best quality images.
This paper proposed a multianalytical, non-invasive, accessible and expensive (compared to traditional ones) approach for the characterization of pictorial surfaces. A set of 18 pigments and 37 mixtures widely used in the Middle Ages in the Apulia (Italy) artistic and archaeological contexts was considered, and corresponding replicas were produced. Pigments were first characterized by X-ray powder diffraction (XRPD), then replicas were studied by optical microscopy using a portable instrument, colorimetry and fiber optic reflectance spectroscopy (FORS). The investigation provided encouraging results, which allowed the creation of a library including diagnostic microscopic, colorimetric and reflectance markers of each pigment and the identification of the mixtures’ components.
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