1,720,997 research outputs found
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
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
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
A first report on anion vacancies in a defect MgAl2O4 natural spinel
No full textThe chemical and structural features of a natural spinel sensu stricto (s.s.) sample were
studied by a multi-analytical approach, including electron microprobe analysis (EMP), Fourier
transform infrared spectroscopy (FTIR), and single crystal X-ray diffraction structural
refinement (SREF). The sample, coming from impure dolomitic marbles of Pegu (Myanmar),
has an anomalous chemistry with an Mg-content exceeding that of the ideal formula. In
addition, a chemical zoning along a line-scan of EMP analyses was observed, with Mg and
Al amounts showing opposite trends. The comparatively high and low concentrations,
respectively, of divalent and trivalent cations lead to a deficit of positive charges. Thus, the
requirement of neutrality of global charges for crystal structures appears to be violated, in this
case. The possible reasons accounting for the anomalous chemistry are discussed. Based on
the combined EMP, FTIR and SREF results, it is concluded that anion vacancies can
adequately compensate for the observed deficit of positive charges. Thus, the analysed sample
is a defect spinel. This is the first report of anion vacancies in a natural spinel s.s. With
reference to the ideal formula MgAl2O4, the formation of anion vacancies, coupled to an
excess of Mg and a deficiency of Al, may be described by the substitution mechanism
2Mg2++V□→2Al3++O2–, where V□ represents an oxygen vacancy
- …
