1,721,082 research outputs found
Tektites and microtektites iron oxidation state and water content
Full text linkFe redox and water content of impact melt are important parameters as they can greatly affect melt density and viscosity which, in turn, are important parameters greatly affecting the melt evolution and fate. In this manuscript I briefly describe recent research on X-ray absorption spectroscopy (XAS) determination of Fe oxidation state and micro Fourier transform infrared spectroscopy (FTIR) determination of water content putting them in the context of previous research done with different techniques. In comparison with other techniques requiring large amount of samples (e.g. potassium dichromate titration, Mossbauer spectroscopy, Karl Fischer titration), XAFS and micro FTIR techniques allow to study both macroscopic samples of tektites as well as smaller microtektites giving the same error, making it possible to compare results between tektites and micro-tektites and, consequently, also to find differences between tektites and microtektites or between microtektites from different strewn fields. A brief introduction on impact cratering and melt formation during impact events is aimed to introduce the subject to non specialised readers
Terre rare: le vitamine dell'industria moderna
No full text“Se il Medio Oriente ha il petrolio, la Cina ha le terre rare”: lo disse nel 1992 il leader Deng Xiaoping. Oggi i metalli tecnologici sono essenziali per l’industria moderna: ce ne servono 125.000 tonnellate l’anno. Un futuro incerto tra monopolio della produzione e scarse possibilità di riciclo.
Domanda crescente. Una produzione quasi del tutto nelle mani della Cina. Margini di riciclo molto limitati. Questa la fotografia attuale del mercato delle terre rare, i “metalli tecnologici”, essenziali per un’ampia gamma di applicazioni: smartphone, microfoni, veicoli elettrici e ibridi, cuffie, auricolari, macchine a raggi X, solo per dirne alcune.
Tuttavia se la domanda di terre rare, così come il loro prezzo, è soggetta a forti oscillazioni, le loro possibilità di riutilizzo sono ancora quasi nulle a causa della bassa resa e degli alti costi del processo di recupero.As leader Deng Xiaoping said back in 1992 “There is oil in the Middle East; there is rare earth in China.” Technology metals are now essential for modern industry which uses 125,000 tonnes a year. However, the future is uncertain due to production monopolisation and lack of recycling.
Demand is increasing. Production is almost completely dominated by China. Recycling margins are inadequate. This is the current context of the market of Rare Earth Elements (REE) or “technology metals” which are essential for a wide range of applications, from smartphones, to microphones, electric and hybrid vehicles, headphones, earphones and X-ray machines, to name just a few.
However, while the demand for Rare Earth Elements, and their price, is subject to significant fluctuations, reuse is virtually impossible because of their low return and the high costs of the recovery process
Seminari "Distinguished Lectures" dell Societa' Geologica Italiana e Societa' Italiana Mineralogia e Petrologia
No full textLa Geologia del nostro sistema solare:
Quasicristalli, meteoriti, ed esplorazione planetari
Al-Fe disorder in synthetic epidotes: a single-crystal X-ray diffraction study
No full textSynthetic epidotes were produced using a Tuttle type hydrothermal vessel in the temperature range from 500 to 700 degrees C and at 4.5 to 5.0 kbars pressure. Single-crystal X-ray diffraction structure refinements yielded intracrystalline cation distributions for two crystals grown at 600 degrees C (0.68 and 0.73 Fe apfu) and five crystals grown at 700 degrees C (0.88 to 1.08 Fe apfu). The resulting Fe occupancies were compared with those calculated according to a thermodynamic model: The samples formed at 700 degrees C display a Fe distribution between M3 and M1 sites that match the calculation; nevertheless, we found the presence of a small but appreciable amount of Fe in the M2 sites that increases with the total Fe content (up to 0.08 Fe apfu). The crystals formed at 600 degrees C show a much higher disorder than expected, bur have no Fe in the M2 site. Crystal-chemical features of the samples studied were compared with those of a previously published set of natural epidotes displaying a much lower Fe disorder: In particular, the unit-cell volume vs. Fe content relationship is not affected by the intracrystalline Fe distribution. This relation suggests that the Al-Fe intracrystalline distribution is independent of pressure
[4]Fe3+-O distance in synthetic kimzeyite garnet
No full textA synthetic kimzeyite analogue (Ca3Zr2[Fe2SiO12]) has been analysed by powder X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) in order to determine the [4]Fe3+–O distance, for which only few data are available. The XRD-determined cell parameter (a0 = 12.625 ± 0.001 Å) is consistent with those found in the literature for synthetic samples of similar composition. Also interatomic distances (= 2.515 Å, = 2.093 Å, = 1.774 Å, as determined by Rietveld refinement) are in keeping with structural data of natural samples when taking into account chemical differences of the samples examined. Due to the large Fe content of the tetrahedral site, the distance is unusually long compared with those of garnets where tetrahedra are occupied solely by Si, it is instead consistent with structural data for natural kimzeyite.
The XANES data indicate the presence of trivalent Fe in tetrahedral coordination. The EXAFS-derived distance (1.85 ± 0.01 Å) is in agreement with the few literature data available for [4]Fe3+, i.e. tetra-ferriphlogopite ( = 1.86 ± 0.01 Å), rodolicoite ( = 1.825 Å) and silicate glasses (Fe–O = 1.85 ± 0.01 Å and 1.84 ± 0.02 for phonolitic and rhyolitic glasses, respectively). In view of the large size difference between FeO4 and SiO4 tetrahedra a further Rietveld structural refinement was performed assuming a splitting of the oxygen position, resulting in two distinct oxygen sites (OA and OB with fractional oxygen occupancies of 2/3 and 1/3) at 1.845 and 1.606 Å distance, respectively, from the tetrahedral cation.
Although there are still open questions on the distribution of FeO4 and SiO4 tetrahedra and on how the structure accommodates the size difference of these two tetrahedra, this study provides a direct determination of the [4]Fe3+–O distance for which only few data are available in the literature
Rietveld refinement of okayamalite, CA2SiB2O7: structural evidence for the B/Si ordered distribution
No full textThe structure of okayamalite from Arendal, Norway, was refined using the Rietveld method, with CoKα powder X-ray diffraction data (R(F) = 3.69%). Okayamalite exhibits a melilite-type structure, space group P4̄21m, with cell edges a = 7.1248(2), c = 4.8177(2) Å. Si and B are ordered on the T1 and T2 sites respectively, in agreement with the refined tetrahedral distances ( = 1.657 Å and =1.498 Å). In comparison with the other melilite-type compounds, the cation population in okayamalite leads to the minimum structural misfit between tetrahedral and square-antiprism layers
The valence and speciation of sulphur in glasses by X-ray absorption spectroscopy
No full textThe geochemical behavior of sulfur in magmas depends strongly on the oxidation state of sulfur, but this is not easily determined by standard analytical methods. We have measured XANES absorption spectra at the sulfur K-edge and have found that such measurements are useful to characterize the oxidation state and speciation of sulfur in silicate glasses of geological relevance. Measured spectra of a set of reference minerals show the effects of different oxidation states and coordination numbers of sulfur; there is a large shift in energy (~10-12 eV) of the sulfur K-edge between S2- and S6+. This large and easily detectable difference makes possible the measurement of the valence of sulfur in unknown samples by measuring the shift in energy of the absorption edge. This approach is applicable to both crystalline and glassy materials, and useful results have been obtained on samples with as little as 450 ppm S. We have used XANES measurements to characterize oxidation state and speciation of sulfur in a set of natural and synthetic sulfur-bearing glasses. The samples cover a range of composition from basaltic to almost rhyolitic, and some were synthesized over a range of pressure, temperature and oxygen fugacity; glass S content varies between 450 and 3000 ppm. XANES analyses, carried out in fluorescence mode at LURE, allowed determination of the sulfur oxidation state in all of the samples and clearly show that some samples contain a mixture of S2- and S6+; no other sulfur species were observed. Quantitative determination of the abundance of sulfide and sulfate shows good agreement with independent measurements based on electron-microprobe determination of the wavelength shift of sulfur Kα X-rays
Aluminium coordination in tektites: a XANES study
No full textAl K-edge XANES spectra were recorded for six tektites to obtain information on the local structure around Al. Albite, grossular, and andalusite were used as reference materials for Al in fourfold, sixfold, and five+sixfold coordination, respectively. The spectra of the tektites are very similar, except for the intensity of the main absorption edge. Comparison of the spectral features of the tektite glasses with those of the reference materials indicates that Al is tetrahedrally coordinated
Al coordination and local structure in minerals: XAFS determinations and multiple scattering calculations for K-feldspars
No full textThe Al K-edge spectra of three potassium feldspars KA1Si(3)O(8): sanidine, microcline and orthoclase, all with four-fold-coordinated Al, have been measured by X-ray absorption near-edge structure spectroscopy and calculated according to the multiple-scattering formalism using clusters of different sizes and the X-alpha exchange-correlation potential; Although these structures are quite complex, and clusters as large as containing ca. 150 atoms are needed to calculate them, a significant agreement has been obtained between experimental spectra and calculated ones up to 60 eV above the threshold, both for energy positions and for relative intensities. Moreover, while the gross features of all spectra appear to be dominated mainly by the first coordination sphere around the absorber, effects due to higher shells are detected
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