Gioacchino Tempesta

The 6H-SiC structure model: Further refinement from SCXRD data from a terrestrial moissanite

Abstract The crystal structure of a terrestrial 6H-SiC moissanite has been refined in the P63mc S.G. from area detector single crystal X-ray data, down to an R-index on the observed reflections of 0.0205. The cell parameters refined 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, defined as the distances along [0001] between Si-Si layers, which may occur either in cubic (c) or hexagonal (h) configurations, 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 significant distortion of 0.15° is recorded. Finally, the anisotropic displacement factors are utterly very small, identical among different atoms within the experimental error, and significantly 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 configuration links a bilayer in antiparallel, hexagonal configuration. As far as we know this is the first crystal structure refinement of a natural 6H-SiC moissanite. Possible implications on the polytype stability in the light of these results are briefly discussed.

“Silicified” pyrochlore from nepheline syenite (mariupolite) of the Mariupol Massif, SE Ukraine: A new insight into the role of silicon in the pyrochlore structure

Abstract Pyrochlore-supergroup minerals containing relatively high Si concentration are quite common in various geochemical parageneses, e.g., carbonatites, alkaline syenites, pegmatites. However, the role of Si and the mechanism of its incorporation into the structure of these minerals, although widely discussed, have not been explained definitively. Our paper reports the results of comprehensive SEM, EPMA, XRD, TEM, and MAS-NMR studies performed for the first time on a natural pyrochlore, which is the late-magmatic to early hydrothermal accessory component of the nepheline syenite in the alkaline Mariupol massif in Ukraine. It represents partly metamict, patchy-zoned, A-cation depleted, REE-, U-, and Th-bearing fluornatropyrochlore, locally exceptionally rich in SiO2 (up to 13.01 wt%) that underwent both primary and secondary alterations, leading to kenopyrochlore or hydropyrochlore species. The primary alteration was induced by high-temperature, Ca2+- and Si4+-rich, and F- moderate fluids, which affected only some domains of the pyrochlore crystals and resulted in filling the A site vacancies mainly by Ca2+, but also Mn2+, Sr2+, and K+. The secondary alteration, induced by the exposure of the host rock to ground water driving fluid-mediated coupled dissolution-reprecipitation process, affected the whole pyrochlore crystals (both Si-enriched and Si-free domains) and caused, among others, the leaching of some A- and Y-site components. TEM investigations indicate that the selected-area electron diffraction patterns taken from Si-poor areas show strong and sharp diffraction spots related to well-crystalline pyrochlore, whereas the Si-rich areas show weaker spots with a diffuse diffraction halo that are typical of metamict material. Due to the fact that no intergrowth with other Si-bearing phases was observed in the TEM images even at very high magnification, it might be concluded that Si4+ can occupy severely a-decay damaged and chemically altered portions of this structure. The absence of Si in the sixfold-coordinated B site has been corroborated both by compositional relationships, and by the lack of any [6]Si4+ signal around -200 ppm in the MAS-NMR spectrum. A broad signal in the spectrum appearing at around -84 ppm, points to an amorphous species with tetrahedrally coordinated Si, close to Q(2) species defined as Si atom with two bridging O atoms, i.e., [Si(OSi)2(-)2], in the form of finite-length chain-like structures, located in the damaged A and B sites of the primary structure.

XRDT Study of Structural Defects of 6H-SiC Crystals

X-Ray Diffraction Topography (XRDT) and Optical Microscopy (OM) are adopted to study extended structural defects in 6H-SiC bulky crystals. Topographs are taken by means of White Beam Synchrotron Radiation Source (WB-SRS-XRDT) and by means of monochromatic radiation (MoKα1) with conventional source (Lang method). All studied samples are characterised by the presence of linear defects, dislocations and microchannels, uniformly distributed in the crystal. Such defects draw a net of independent systems of parallel lines, with different orientation and different contrast widths. Micro-channels are parallel to the c axis, whereas dislocations are perpendicular or nearly parallel to the c axis. The last are unit screw dislocations. It has been concluded that the growth mechanism is driven by screw dislocations and that channels results from the coalescence of parallel dislocations.

Growth and post-growth defects in a diamond from Finsch mine (South Africa)

A diamond from the Finsch mine (South Africa) was investigated by X-ray diffraction topography to characterize the extended defects and to reconstruct the growth history. The sample was a doubly polished triangular diamond slice with a rounded side in which re-entrant corners could be seen and the specimen exhibited anomalous birefringence. X-ray topographs show that the sample is actually a mosaic crystal that developed through aggregation of sub-individual parts during growth: some slightly rotated relative to one another and others twinned. The twin law is by reticular merohedry and consists of the well-known 2-fold rotation around [111]. Micro-Raman spectroscopy data revealed that the main inclusions are pyrope and enstatite trapped in the inner part of the individual. No dislocations nucleated from the silicate inclusions. Growth-sector boundaries, dislocations, grain boundaries and twinning indicate that these features formed during the growth of diamond. Throughout the whole sample and chiefly in the regions showing a lower density of inclusions, packed lamination lines parallel to an octahedral face can be observed. These laminations represent a polysynthetic twinning commonly observed in diamonds which have undergone plastic deformation and are considered to be a post-growth defect. The analysis of the structural defects allows a reconstruction of a complex growth history, characterized by a sequence of alternating episodes of growth and dissolution and, lastly, plastic deformation. This study represents a methodological contribution to the studies on the origin of diamond.

XRDT and TEM Study of Defects and Polytypism in Natural Moissanite and Synthetic SiC Crystals

Structural defects and degree of order of natural and synthetic moissanite have been investigated by XRD Topography and TEM. XRDT analyses of synthetic 6H-SiC wafers allowed to study extended defects and to identify and localize coalescence of polytypes. The observed linear defects are microchannels and dislocations. Axial screw dislocations, either parallel or slightly inclined to the c-axis, suggest that the growth mechanism for the bulk crystals was mainly by spiral growth. Moreover, the study of line orientations of the dislocations allow to reconstruct the growth sector evolution of the sample. Therefore, the convex shape of the growing surface is attained by the development of growth sectors (10l) neighbouring growth sector (001). The coalescence of a thin lamella of a different polytype has been also localized and investigated. The contrast analysis and the diffraction pattern of the lamella are consistent with a 15R-SiC crystal. Such coalescence indicates local variations of growth conditions. TEM images and selected area electron diffractions (SAEDs) strongly differentiate natural from synthetic samples. SAED patterns with [010] incidence of natural crystals are consistent with the 6H polytype and do not show streaks along the [001] stacking direction. Synthetic samples are comparatively much more disordered. Conventional images show high density of (001) faults, not observed in natural samples. Consistently, SAED patterns of the [100] zone are streaked along c*. High resolution (HR) imaging shows that synthetic samples mainly consist of (001) stacking sequence described as (32)3. Locally mixed stacking sequence described by notation 23(3233)5, probably referred to a long period polytype, are present.

Non-destructive, multi-method, internal analysis of multiple inclusions in a single diamond: First occurrence of mackinawite (Fe,Ni)1+xS

Abstract A single gem lithospheric diamond with five sulfide inclusions from the Udachnaya kimberlite (Siberia, Russia) has been analyzed non-destructively to track the growth conditions of the diamond. Sulfides are the most abundant mineral inclusions in many lithospheric diamond crystals and are the most favorable minerals to date diamond crystals by Re-Os isotope systematics. Our investigation used non-destructive, micro-techniques, combining X-ray tomography, X-ray fluorescence, X-ray powder diffraction, and Raman spectroscopy. This approach allowed us to determine the spatial distribution of the inclusions, their chemical and mineralogical composition on the microscale, and, finally, the paragenetic association, leaving the diamond host completely unaffected. The sample was also studied by X-ray diffraction topography to characterize the structural defects of the diamond and to obtain genetic information about its growth history. The X-ray topographic images show that the sample investigated exhibits plastic deformation. One set of {111} slip lamellae, corresponding to polysynthetic twinning, affects the entire sample. Chemical data on the inclusions still trapped within the diamond show they are monosulfide solid solutions of Fe, Ni and indicate a peridotitic paragenesis. Micro-X-ray diffraction reveals that the inclusions mainly consist of a polycrystalline aggregate of pentlandite and pyrrothite. A thorough analysis of the Raman data suggests the presence of a further Fe, Ni sulfide, never reported so far in diamonds: mackinawite. The total absence of any oxides in the sulfide assemblage clearly indicates that mackinawite is not simply a “late” alteration of pyrrhotite and pentlandite due to secondary oxidizing fluids entering diamond fractures after the diamond transport to the surface. Instead, it is likely formed as a low-temperature phase that grew in a closed system within the diamond host. It is possible that mackinawite is a more common phase in sulfide assemblages within diamond crystals than has previously been presumed, and that the percentage of mackinawite within a given sulfide assemblage could vary from diamond to diamond and from locality to locality.

Refinement of the crystal structure of a terrestrial 6H-SiC

In course of a detailed analysis of various intermetallic compounds derived from the AlB2 type structure, highresolution X-ray diffraction on single crystals proved very powerful in deriving precise composition of individual phases. E.g., the homogeneity ranges of EuGa2 and SrGa2 were determined and associated structure distortions induced by changes in composition were analyzed. The structural models derived are also discussed on the basis of solid state NMR data and in comparison to quantum chemical calculations. m20.p05