1,721,023 research outputs found
A Mössbauer and FTIR study of synthetic amphiboles along the magnesio-riebeckite – ferri-clinoholmquistite join
No full textCharacterization of synthetic hedenbergite (CaFeSi2O6)–petedunnite (CaZnSi2O6) solid solution series by X-ray single crystal diffraction
No full textClinopyroxenes along the solid solution series hedenbergite (CaFeSi2O6)-petedunnite (CaZnSi2O6) were synthesized under hydrothermal conditions and different oxygen fugacities at temperatures of 700 to 1200 °C and pressures of 0.2 to 2.5 GPa. Properties were determined by means of X-ray diffraction, electron microprobe analysis and 57Fe Mössbauer spectroscopy at 298 K. Unit-cell parameters display a linear dependency with changing composition. Parameters a0 and b0 exhibit a linear decrease with increasing Zn content while the monoclinic angle β increases linearly. Parameter c0 is not affected by composition and remains constant at a value of 5.248 Å. The molar volume can be described according to the equation Vmol (ccm mol-1)=33.963(16)-0.544(31)*Zn pfu. The isomer shifts of ferrous iron on the octahedral M1 site in hedenbergite are not affected by composition along the hedenbergite-petedunnite solid solution series and remain constant at an average value of 1.18 mm s-1. Quadrupole splittings of Fe2+ on the M1 are, however, strongly affected by composition, and they decrease linearly with increasing petedunnite component in hedenbergite, ranging from 2.25 mm s-1 for pure hedenbergite end member to 1.99 mm s-1 for a solid solution containing 84 mole% petedunnite. The half-widths of intermediate solid solutions vary between 0.26 and 0.33 mm s-1, indicating, in accordance with the microprobe analyses and X-ray diffraction, that samples are homogeneous and well-crystallized. The data from this study demonstrate that the crystallinity of hedenbergitic clinopyroxenes can be improved by using oxide mixtures as starting materials. Crystal sizes for intermediate compositions range up to 70 μm, suitable for standard single-crystal X-ray analysis
Single-crystal polarized FTIR spectroscopy and neutron diffraction refinement of cancrinite
No full textWe relate a single-crystal FTIR (Fourier transform infrared) and neutron diffraction study of two natural cancrinites. The structural refinements show that the oxygen site of the H(2)O molecule lies off the triad axis. The water molecule is almost symmetric and slightly tilted from the (0001) plane. It is involved in bifurcated hydrogen bridges, with Ow center dot center dot center dot O donor-acceptor distances >2.7 angstrom. The FTIR spectra show two main absorptions. The first at 3,602 cm(-1) is polarized for E perpendicular to c and is assigned to the nu(3) mode. The second, at 3,531 cm(-1), is also polarized for E perpendicular to c and is assigned to nu(1) mode. A weak component at 4,108 cm(-1) could possibly indicate the presence of additional OH groups in the structure of cancrinite. Several overlapping bands in the 1,300-1,500 cm(-1) range are strongly polarized for E perpendicular to c, and are assigned to the vibrations of the CO(3) group
X-ray diffraction as a function of temperature for Fo92-Fa8: implications for the hermean surface.
No full textIt is well known that the surface of Mercury shows
temperature variations extremely large at specific
locations [1]. Such a range can cause strong crystal
structure changes and, as a consequence, the detected
spectral signature could be significantly affected.
Here we investigated the low-high temperature
behaviour of one of the olivine, which has been
recently considered as a possible constituent of the
Mercury surface [2]. Aim of the work is to determine
which expansion the olivine undergoes within a
temperature range typical of the Mercury surface in
order to verify which could be the effect of thermal
expansion on a spectra
High-pressure phase transformation in LiFeGe2O6 pyroxene
No full textA synthetic pyroxene with composition LiFeGe2O6 and space group P21/c at ambient conditions was investigated by single-crystal X-ray diffraction using a diamond anvil cell. The unit-cell parameters and crystal structure were determined at eight different pressures up to 8.7 GPa. Between 4.16 and 4.83 GPa, the sample shows a strongly first-order phase transition as indicated by a drastic drop in a, c, β, and unit-cell volume. The transition is marked by the disappearance of b-type reflections (h + k = odd) forbidden in a C-centered lattice. The volume bulk modulus of the P21/c phase is estimated to be 110 GPa as compared to 147 GPa of the C2/c one. The crystal structure evolution as a function of pressure is mainly influenced by the kinking of tetrahedral chains; the A and B non-symmetry equivalent chains of the P21/c phase undergo strong deformations up to 4.16 GPa (A chain ~2%, B chain ~5.3%). At the transition, the two chains become symmetry equivalent and the single tetrahedral chain of the C2/c phase shows only minor deformations with pressure (~1.9%) due to its already strong kinking (~130°). Such behavior is the main reason for the strong difference in compressibility between the low- and high-symmetry forms
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Crystal-chemical characterization of fibrous amphiboles by FTIR and Mössbauer spectroscopies
No full textThe mechanism through which asbestos amphiboles may give rise to mesothelioma is still not completely understood. Several factors have been taken into account, and these include the morphological aspect of the fiber, the chemical composition, and a variety of surface properties which are ultimately responsible for the mineral-cellule interactions[1]. Studies in vitro demonstrate that the morphology of the fiber has a strong role in determining its biological danger, because very thin and long crystallites are hardly eliminated by the alveolar macrophages. However, recent studied show that lung injury after asbestos exposure is associated with an oxidative stress that is catalyzed by iron in the fiber[2]. It follows that the rapid determination of the chemical composition of asbestiform amphiboles, and of its Mg/Fe ratio in particular, is extremely important in environmental studies. The direct analysis can be achieved only by micro-chemical tools such as by EDS- or WDS-equipped electron microscopes. However, these techniques are extremely expensive and often unsuitable when dealing with extremely fibrous (diameter < 3 μm) materials. Therefore a rapid and easily accessible method is highly desirable in large scale environmental monitoring. The best alternative to EMPA is provided by FTIR spectroscopy, a technique which can be used on both single crystals (down to few μm in dimension) and powders. Here we present the results of a new calibration based on the analysis of a large set of well-characterized fibrous and prismatic natural amphiboles spanning a very large variety of chemical compositions and geological occurrences. All samples were previously studied using X-ray diffraction and EMPA. FTIR spectra in the principal OH-stretching region were collected on KBr disks prepared with a mineral:matrix = 5:150 mg ratio. Most spectra show four prominent bands which can be assigned to the combination of Mg and F2+ at the OH-coordinated M(1,3) sites[3,4,5,6]. The digitized spectra were fitted by interactive optimization followed by least-squares refinement; all bands were modelled as symmetric Gaussians. [3,4] showed that the binary site-occupancies at M(1) and M(3) in the amphibole structure are related to the observed intensities of the four (A to D) components in the principal IR OH-stretching spectrum. Using the original equations of [7]:
M(1,3)Mg = 3IA + 2IB + IC and M(1,3)M2+ = IB + 2IC + 3ID (with M2+ = Fe2+)
where IA-ID are the intensities measured for the corresponding A to D bands, one can derive the (Mg, M2+) site populations at M(1,3) with a high degree of confidence. This method is particularly suitable for asbestiform materials which cannot be properly characterized by EMP. The present work shows that the above spectroscopic tool can be applied to a large variety of amphibole types. For species were significant (Mg, Fe) are present at M(4) (i.e. anthophyllite–cummingtonite–grunerite) an additional information (e.g. Mössbauer) is however required for a complete characterization of the sample.
[1] van Oss, C.J., Naim, J.O., Costanza, P.M., Giese, R.F. Jr., Wu, W., Sorling, A.F. (1999) Clays and Clay Minerals, 47, 697-707.
[2] Xinchao, W., Wu, Y., Stonehuerner, J.G., Dailey, L.A., Richards, J.D., Jaspers, I., Piantadosi, C.A., Ghio, A.J. (2006) Am. J. Respir. Cell. Mol. Biol., 34, 286–292.
[3] Della Ventura, G., Robert, J.-L., Hawthorne, F.C. (1996) Geochimica and Cosmochimica Acta, vol. spec. 5, 55-63.
[4] Della Ventura, G., Robert, J.-L., Raudsepp, M., Hawthorne, F.C., Welch, M. (1997) American Mineralogist, 82, 291-301.
[5] Iezzi, G., Della Ventura, G., Hawthorne, F.C., Pedrazzi, G., Robert, J.-L., Novembre, D. (2005) European Journal of Mineralogy, 17, 733-740.
[6] Iezzi, G., Della Ventura, G., Bellatreccia, F., Lo Mastro, S., Gunther, M., Bandly, (2007) Mineralogical Magazine, in press.
[7] Burns, R.G. and Strens, R.G.J. (1966) Science, 153, 890-892
Synthesis and crystal-chemistry of alkali amphiboles in the system Na2O-MgO-FeO-Fe2O3-SiO2-H2O as a function of fO2
No full textThis paper reports the results of hydrothermal synthesis in the system Na2O-MgO-FeO-Fe2O3-SiO2- H2O. Four samples of stoichiometric magnesioriebeckite composition, ideally square 3Na(2)Mg(3)Fe(2)(3+)O(22)(OH)(2), were run at 700-800 degrees C, 0.4 GPa, and redox conditions varying from NNO (Nickel-Nickel Oxide) to NNO + 2.3 log fo(2) Powder XRD and SEM-EDX show a high (> 85%) amphibole yield for all samples; however, in no case was the end-member composition attained. EMP analyses show that the amphiboles obtained deviate strongly from nominal stoichiometry toward magnesio-arfvedsonite [NaNa2Mg4Fe3+Si8O22(OH)(2)]. Powder XRD patterns were indexed in the space group C2/m; refined cell-parameters reflect variations in the amphibole composition, and the cell volume is correlated linearly with the A-site occupancy. Mossbauer spectra show that in all samples, Fe3+ is completely ordered at M2, whereas Fe2+ occurs at the M1, M3, and M4 sites. The Fe3+/Fe2+ ratio is a function of f(o2): for increasing oxidation conditions, there is significant increase in Fe-M2(2+) and decrease in Fe2+, notably in Fe-M4(2+). Mossbauer spectra also show significant variation in Fe-M1(2+) and Fe-M3(2+) quadrupole splitting as a function of the Fe3+ content in the amphibole. IR spectra in the OH-stretching region show a well-resolved quadruplet at frequencies < 3680 cm(-1), assigned to octahedral (M1.3)(Mg, Fe2+)-OH-(A)square configurations, and a broad band consisting of four overlapping components related to (M1,3)(Mg, Fe2+) configurations associated with occupied A-sites. Quantitative evaluation of the relative band intensities suggests a linear increase of A-site occupancy with decreasing fo(2) of synthesis. The composition of the amphiboles synthesized, can be best described by a combination of the (C)(Mg,Fe2+)(1) (B)(Mg,Fe2+) Fe-C(-1)3+ Na-B(-1) and the Na-A(1) (C)(Mg,Fe2+)(1) (A)square(1) Fe-C(-1)3+ exchange vectors. The experimental trend is in accord with the trend documented for natural amphiboles, and suggests that the amphibole composition can in fact be used to monitor changes in f(o2) during crystallization
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