204 research outputs found
Intermediate range order in (Fe,Al) silicate network glasses: a neutron diffraction and EPSR modeling investigation
The local structural environment and the spatial distribution of iron and
aluminum ions in sodosilicate glasses with composition NaFexAl1-xSi2O6 (x = 1,
0.8, 0.5 and 0) is studied by high-resolution neutron diffraction combined with
structural modeling using the Empirical Potential Structure Refinement (EPSR)
code. This work gives evidence of differences in the structural behavior of
Al3+ and Fe3+, which are both often considered to act as network formers in
charge-balanced compositions. The short-range environment and the structural
role of the two cations are not composition dependent, and hence the structure
of intermediate glasses can then be seen as a mixture of the structures of the
two end-members. All Al3+ is 4-coordinated for a distance
d[4]Al3+-O=1.760.01{\AA}. The high-resolution neutron data allows
deciphering between two populations of Fe. The majority of Fe3+ is
4-coordinated (d[4]Fe3+-O=1.870.01{\AA}) while the remaining Fe3+ and all
Fe2+ (~12% of total Fe) are 5-coordinated (d[5]Fe-O=2.010.01{\AA}). Both
AlO4 and FeO4 are randomly distributed and connected with the silicate network
in which they share corners with SiO4 tetrahedra, in agreement with a
network-forming role of those species. On the contrary FeO5 tends to form
clusters and to share edges with each other. 5-coordinated Fe is interpreted as
network modifier and it turns out that, even if this coordination number is
rare in crystals, it is more common in glasses in which they can have a key
role on physical properties
Structural relaxation around substitutional Cr3+ in MgAl2O4
The structural environment of substitutional Cr3+ ion in MgAl2O4 spinel has
been investigated by Cr K-edge Extended X-ray Absorption Fine Structure (EXAFS)
and X-ray Absorption Near Edge Structure (XANES) spectroscopies.
First-principles computations of the structural relaxation and of the XANES
spectrum have been performed, with a good agreement to the experiment. The Cr-O
distance is close to that in MgCr2O4, indicating a full relaxation of the first
neighbors, and the second shell of Al atoms relaxes partially. These
observations demonstrate that Vegard's law is not obeyed in the MgAl2O4-MgCr2O4
solid solution. Despite some angular site distortion, the local D3d symmetry of
the B-site of the spinel structure is retained during the substitution of Cr
for Al. Here, we show that the relaxation is accomodated by strain-induced bond
buckling, with angular tilts of the Mg-centred tetrahedra around the Cr-centred
octahedron. By contrast, there is no significant alteration of the angles
between the edge-sharing octahedra, which build chains aligned along the three
four-fold axes of the cubic structure.Comment: 7 pages, 4 figure
Современные тенденции развития рекреационного хозяйства в Украине
Chromiferous ultramafic rocks from Campo Formoso (Bahia State, Brazil) exhibit various stages of hydrothermal alteration and weathering, each being characterized by distinct chromiferous parageneses. These Cr-bearing minerals are mainly phyllosilicates, but other phases are also present as chromiferous opal and hydroxycarbonates. Optical absorption spectra and K-edge fine structure allow a better knowledge of chromium crystal chemistry in these minerals. The first stages of chromite oxidation have been precised, and particularly the influence of structural factors was pointed out : the inversion grade of these spinels controls the evolution from a mainly direct chromite to the inverse forms (ferritchromit). Chromium ions inserted in mineral structures do not exhibit any change in the oxidation state during alteration processes. In contrast, crystal field stabilization energy (CFSE) of Cr3+ ions exhibits important variations among the studied minerals. Study of EXAFS spectra demonstrated that Cr-O distances do not change significandy among the investigated minerals, the effective oxygen charge playing the main role in the observed CFSE variations. This latter parameter explains the concentration of chromium in these minerals and contributes to the stability of some phases like clinochlores and hydroxycarbonates, where trivalent chromium has the highest CFSE.Les ultrabasites chromifères de Campo Formoso (Etat de Bahia, Brésil) montrent plusieurs stades successifs d'altération hydrothermale puis superficielle qui s'accompagnent de paragenèses chromifères variées. Les minéraux caractéristiques de ces dernières sont essentiellement des phyllosilicates, mais on note aussi d'autres phases comme la stichtite et l'opale chromifère. Les spectres d'absorption X ont permis de caractériser la cristallochimie du chrome dans ces minéraux. Les premiers stades d'oxydation des chromites ont été précisés, en particulier l'influence de la structure réelle du spinelle (degré d'inversion) qui contrôle les conditions d'évolution de la chromite. Le degré d'oxydation +3 du chrome est conservé lors des processus d'altération et seule l'énergie de stabilisation liée au champ cristallin montre des changements importants. L'étude des spectres EXAFS (Extended X-ray Absorption Fine Structure) a montré que, les distances Cr-O ne changeant pas entre les minéraux étudiés, seule la nature des ligandes a une influence sur la valeur de l'énergie de champ cristallin. Ceci explique la forte concentration en chrome de ces minéraux d'altération et contribue à la stabilité des phases comme les clinochlores ou les hydroxycarbonates où les ions Cr3+ ont la plus forte énergie de stabilisation.Calas Georges, Manceau Alain, Novikoff André, Boukili Hassan. Comportement du chrome dans les minéraux d'altération du gisement de chromite de Campo Formoso (Bahia, Brésil). In: Bulletin de Minéralogie, volume 107, 6, 1984. pp. 755-766
XANES determination of chromium oxidation states in glasses: comparison with optical absorption spectroscopy
The oxidation state of chromium in glasses melted in an air atmosphere with
and without refining agents was investigated by Cr K-edge X-ray Absorption
Near-Edge Structure (XANES) and optical absorption spectroscopy. A good
agreement in the relative proportion of Cr(III) and Cr(VI) is obtained between
both methods. We show that the chemical dependence of the absorption
coefficient of Cr(III) is less important in XANES than in optical absorption
spectroscopy. The comparison of glasses melted under different conditions
provides an indirect assessment of the molar extinction coefficient of Cr(VI)
in glasses
Structural relaxation around substitutional Cr3+ in pyrope garnet
The structural environment of substitutional Cr3+ ion in a natural pyrope
Mg3Al2Si3O12 has been investigated by Cr K-edge Extended X-ray Absorption Fine
Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) coupled with
first-principles computations. The Cr-O distance is close to that in
knorringite Mg3Cr2Si3O12, indicating a full relaxation of the first neighbors.
The local C3i symmetry of the octahedral Y site is retained during Cr-Al
substitution. The second and third shells of neighbors (Mg and Si) relax only
partially. Site relaxation is accommodated by strain-induced bond buckling,
with angular tilts of the Si-centered tetrahedra around the Cr-centered
octahedron, and by a radial deformation of the Mg-centered dodecahedra
Crystal structure of synthetic Mg3Cr2Si3O12, the high-pressure Cr end-member of the knorringite-pyrope garnet series
Knorringite, the Cr-end-member of the pyrope garnet series (Nixon et al.
1968), often occur in high proportions in kimberlite garnets and is thus used
for tracing high-pressure deep-earth conditions favorable to the formation of
diamonds, in which knorringite-rich garnet can occur as inclusions. However,
although the synthesis of knorringite is reported in the literature (Ringwood
1977; Irifune et al. 1982; Taran et al. 2004), the structure of the pure
end-member has not been yet determined from experimental data. In this study,
the crystal structure of knorringite, Mg3Cr2(SiO4)3, has been refined from high
resolution synchrotron X-ray powder diffraction data recorded under ambient
conditions on a polycrystalline sample synthesized at 12 GPa in a multi-anvil
apparatus. The structure is cubic, space group Ia-3d, a = 11.5935(1), V =
1558.27(4) {\AA}3, dcalc = 3.97 g.cm-3. The Cr-O distance of 1.957(2) {\AA} is
consistent with EXAFS results on the same sample. This short distance indicates
a substantial compression of the CrO6 octahedron, compared to ambient pressure
Cr3+-minerals such as uvarovite ( = 1.99 {\AA}, Andrut and Wildner 2002).
Our experimental results thus confirm early empirical predictions based on
series of high-pressure Cr-garnet end-members (Fursenko 1981), showing that the
values of the Cr-O distance and the Cr-O-Si angle decrease with the
augmentation of pressure and with the diminution of the size of the divalent
cation
Environmental Mineralogy: New challenges, new materials
The close links between mineralogy and materials science are leading to major developments in how society deals more effectively with energy and environmental challenges. The fast expanding field of "environmental mineralogy" helps mitigate major environmental issues related to the impact of anthropic activities on the global ecosystem. Focusing on energy-related materials and environmental cleanup, this article shows how minerals inspire us to design new materials for advanced technologies needed for energy production, managing contaminated areas, and disposing of nuclear waste. We illustrate the environmental importance of nanomaterials, non- and poorly crystalline phases, and the interactions between minerals and ubiquitous microbial activity
Electron sources for plasma electronics and different technological application
There are the following advantages of applying electron guns with plasma cathodes in devices exciting microwave radiation: stability of their parameters, high density of current, relative insensitivity to ion bombardment and the possibility of operating over a wide range of pressure values of a plasma-generating gas [1-5]. The given work aims at constructing the guns with the parameters necessary for the excitation of microwaves of high amplitudes in the slow-wave structures: the beam energy is 20-30 kV, the current is up to 5 A, and the pulse duration is 0,11÷1 ms. The principal problem arising during construction of heavy-current electron sources with plasma emitters consists in the following: it is necessary to provide such conditions of the gas volume, under which the discharge firing would be stable and the emissive plasma generation be effective, whereas a gas breakdown in the accelerating gap must be eliminated
Direct Evidence for the Co-Expression of URP and GnRH in a Sub-Population of Rat Hypothalamic Neurones: Anatomical and Functional Correlation
Urotensin-II-related peptide (URP) is an eight amino-acid neuropeptide recently isolated from rat brain and considered as the endogenous ligand for the GPR14 receptor. Using single and double immunohistochemical labelling, in situ hybridization and ultrastructural immunocytochemistry, we explored the cellular and subcellular localization of URP in the male rat brain. URP peptide was detected in numerous varicose fibres of the median eminence (ME) and organum vasculosum laminae terminalis (OVLT) as well as in neuronal cell bodies of the medial septal nucleus and diagonal band of Broca where corresponding mRNA were also detected. Combining in situ hybridization with immunohistochemistry, we showed that cell bodies of the rat anterior hypothalamus contained both URP mRNA and GnRH peptide. In addition, double ultrastructural immunodetection of URP and GnRH peptides clearly revealed, in the median eminence, the co-localization of both peptides in the same neuronal processes in the vicinity of fenestrated portal vessels. This remarkable cellular and subcellular distribution led us to test the effect of URP on the GnRH-induced gonadotrophins release in the anterior pituitary, and to discuss its putative role at the level of the median eminence
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