1,721,022 research outputs found
A 94-layer long-period mica polytype: a TEM study
No full textA 94-layer long-period mica polytype was studied by transmission electron microscopy (TEM). This is the longest periodicity found up to now in micas (c ≈ 95.9 nm). It was observed in a fragment of a Mg-rich annite (biotite) crystal from dacite rocks of Džep, Serbia. The crystal region containing it extends about 800 nm along 1/c*.
One-dimensional lattice fringe images obtained by bright-field (BF) illumination allowed identification
of the very long-period polytype. The latter was characterized by selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). It is an inhomogeneous polytype belonging to the subfamily-A, based on the 2M1 structural series. Its 94-layer stacking sequence has been determined. The fringe contrasts of the BF images were correlated to the stacking sequence determined from HR images. The correlation verified that the same sequence occurred eight consecutive times. Analytical electron microscopy (AEM) revealed that the chemical composition of the 94-layer mica polytype is similar to that previously observed in randomly stacked and faulted areas of the same crystal. No remarkable chemical variation occurs between the 94-layer polytype and its adjacent crystal regions, the latter containing non-periodic stacking faults
Cross-twinning in a natural spinel from Sri Lanka
No full textA modified cross-twinning growth mechanism
is put forward to explain the anomalous morphology of a
spinel multiple-twin from Sri Lanka, flattened crosswise
the twin planes. Cross-twinning in spinel was found also in
other specimens from Pegu (Myanmar), and the results
were published in a previous paper. This particular type of
twinning is derived from the combination of cyclic twinning
with lamellar twinning, so that these samples may be
thought of as partial fivelings (cubic cyclic {111} twins
with five components sharing a common pseudofivefold
axis). In the present paper, the sample from Sri
Lanka has been suitably cut with the aim of focusing the
study on the cross-twinning region. The transformation
matrices that link the orientation states of each couple of
twin components have been determined by means of White
Beam Synchrotron Radiation Topography. They showed
that the specimen is made up of four twin components (A,
B, C and D), with three twin planes: (-111)A/B, (1-1-1)B/C
and (-1-1-1)AC/D. They also showed that the cross-twinned
individuals (B and D) actually are not twinned to each
other, and that a simple crystallographic relationship holds
between them. X-ray diffraction topography by conventional
source allowed to image the crossing-region and to
determine that the cross-twinned individuals are in contact
through a semi-coherent boundary, with twinning dislocations
contributing to relieve the coherency strains. Electron
probe microanalyses with wave dispersive spectroscopy
showed that the chemical composition is almost homogeneous,
at least within the spatial resolution limit of this
technique. The similar growth features observed in the
spinel sample from Sri Lanka and in those from Myanmar
are interpreted as growth marks, indicators of a similar
origin: in both cases they are found in impure dolomitic
marbles. In particular, the specimen from Sri Lanka results
from the interaction of thermal and metasomatic effects
due to contact metamorphism. An unusual stepped morphology
of the (1-10)C face close to the (-1-1-1)C/D twin
boundary, possibly due to corrosion and re-growth processes
acted preferentially at a re-entrant corner by metasomatic
fluids, is interpreted as indicator of a metasomatic
event that succeeded to the crystal growth, the latter
occurred by thermal effect
Structural Defects and Polytypism in Moissanite and Synthetic SiC Crystals
No full textIn recent years, defect characterisation of SiC polytypes has
been developed significantly, due to the increasing importance
of such polytypes for high temperature, high-power and highfrequency
electronic devices [1]. On the other side, moissanite-
SiC is a rare mineral that was discovered in 1905 within a
meteorite and only recently as a rock-forming mineral (8.4
vol%) in one unique specimen of a terrestrial volcanic rock [2].
Besides it has been found in kimberlitic pipes and can provide
some light on the redox conditions of the Earth Mantle. Finally
SiC is a particularly interesting species of presolar grain because
the polytype distribution can be related to different parameters
of the expanding stellar atmospheres of asymptotic giant
branch-AGB-stars.
In the frame of a research program in progress on SiC, growth
defects of bulky gem-crystals of 6H-SiC were studied by X-ray
Diffraction Topography-XRDT and it was found that the main
types of defects were dislocations parallel and perpendicular
to the growth directions, and micro-channels [3].
This piece of work is devoted to the study of structural defects
and degree of order of natural and synthetic moissanite.
Natural and synthetic samples have been investigated by XRD
Topography and TEM, focusing on the relationships between
defects and growth conditions. XRDT analyses of synthetic
6H-SiC allowed the characterization of dislocations and channels
to be made and other polytypes for further investigation by
TEM to be localized. All studied sample are characterized by
the presence of linear defects, dislocations and micro-channels,
uniformly distributed in the crystal. Moreover samples grown
by means of Physical Vapour Transport-PVT method, show the
same linear defects with different character, strictly related to
growth conditions.
TEM images and electron diffractions (EDs) strongly differentiate
natural from synthetic samples. ED patterns with [100]
incidence of natural crystals are consistent with the 6H polytype
and do not show streaks along the [001] stacking direction. This
result is confirmed by structure refinement from area detector
single crystal X-ray data [4]. Synthetic samples are comparatively
much more disordered. Conventional images show high
density of (001) faults, not observed in natural samples. Consistently,
ED patterns of the [100] zone are streaked along c*.
Atomic resolution imaging shows that synthetic samples mainly
consist of (001) stacking sequence described as (32)3 [5]. Locally
mixed stacking sequence described by notation 23(3233)5,
probably referred to a long period polytype, are present.
[1] Neudeck, P. G., Journal of Electronic Materials, 1995, 24, 283.
[2] Di Pierro S., Gnos E., Grobety B. H., Armbruster T., Bernasconi
S. M. and Ulmer P., Am. Mineral., 2003, 88, 1817.
[3] Agrosì G., Fregola R.A., Monno A., Scandale E., Tempesta G.,
Materials Science Forum, 2005, 483-485, 311.
[4] Capitani G.C.*, Tempesta G., Di Pierro S., Scandale E., 2006, This
congress.
[5] Zhdanov G.S., Minervina Z.V., Compt. Rend. Acad. Sci. URSS 1945,
48.
Industrial mineralogy
23rd European Crystallographic Meeting, ECM23, Leuven, 2006
Acta Cryst. (2006). A62, s64
Pag
X-ray diffraction topographic study of twinning and growth of natural spinels
No full textTwo spinel twinned crystals coming from Pegu (Myanmar) have been studied by means of X-ray topographic methods,
optical microscopy and electron microprobe analysis.
Topographs, taken with conventional and synchrotron sources, showed the presence of a twin lamella in both samples as well as
the presence of cross-twinning in one of them. In particular, white beam synchrotron radiation source topography allowed to detect
the twin individuals, to identify and to locate the twin planes, and to determine spatial relationships and mutual orientations of the
twin components. By combining optical and topographic observations, an unusual morphology, flattened crosswise the twin planes
bounding the twin lamella, has been outlined in both samples and related to the growthmechanism.
Microchemical analyses showed that the twin individuals in each sample have nearly identical compositions,whereas the samples
differ mainly for the Mg and Zn contents. These observations suggest that they grew in similar and chemically closed microenvironments.
The results, taken as a whole, indicate that twinning and cross-twinning occurred since nucleation of the samples, and they mark
similar growth conditions
GROWTH MARKS APPLIED TO THE ORIGIN OF GEMMOLOGICAL MATERIALS: GROWTH DEFECT CHARACTERIZATION OF YELLOW SAPPHIRES BY X-RAY DIFFRACTION TOPOGRAPHY
No full textApplication of Laser Induced Breakdown Spectroscopy on gem quality minerals: some cases of study
No full textThe application of the Laser Induced Breakdown Spectroscopy (LIBS) to gemmological materials is well known for the gemstone treatment detection but LIBS can also provide a useful contribution in mineralogical studies in which the chemical analyses is essential, playing a key role in the studies on the origin of minerals (Agrosì et al., 2014). The possibility to detect simultaneously all elements, including also light elements such as boron, lithium, and beryllium in a fast way and without sample preparation its clearly a great advantage of the LIBS. Moreover this technique is able to detect chromophorous elements even if they are in very low concentration. Nevertheless the main problems affecting LIBS are the difficulties to perform quantitative analyses and the partial destructivity of the method. Recently, with the development of the technique and in particular with the development of a new softwares there is also the possibilities to perform quantitative analyses with or without calibration. The last examples are the standard Calibration-Free LIBS and the One Point Calibration LIBS, CF-LIBS and OPC-LIBS respectively (Cavalcanti et al., 2013). The improvement of the instrumentation performance and the possibility to focalize the laser beam with an optical lens of a microscope allow to obtain an high resolution and a low destructivity of the sample. Moreover, the use of a double-pulse laser produce a signal enhancement useful for the quantitative chemical analyses guaranteeing a minimal sample damage. In this way we may obtain a considerable improvement on the detection limit of the trace elements, whose determination is essential to define the origin of gemstones. In the mineralogical studies the μ-LIBS appear very attractive because the possibility to select the smallest sampling areas ensures a better reconstruction of the chemical zoning and consequently allows to stimate the rock forming condition because the chemical zoning is sensitive to the changing of the physical and chemical parameters. Some cases of study have been performed on beryls and corundums with different objectives and the results will be discuss
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