Actual Structure

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Alessandro Guastoni - One of the best experts on this subject based on the ideXlab platform.

  • what is the Actual Structure of samarskite y a tem investigation of metamict samarskite from the garnet codera dike pegmatite central italian alps
    American Mineralogist, 2016
    Co-Authors: Giancarlo Capitani, Enrico Mugnaioli, Alessandro Guastoni
    Abstract:

    We investigated, by scanning and transmission electron microscopy (SEM, TEM), wavelength- and energy-dispersive spectroscopy (WDS, EDS), and electron diffraction tomography (EDT), several (Y-REE-U-Th)-(Nb-Ta-Ti) oxides from the Garnet Codera dike pegmatite (Central Italian Alps). These oxides have compositions in the samarskite-(Y) field and yield an amorphous response from the single-crystal X-ray diffractometer. Backscattered electron images reveal that the samples are zoned with major substitutions involving (U+Th) with respect to (Y+REE). At the TEM scale, the samples show a continuous range of variability both in terms of composition and in radiation damage, and the amount of radiation damage is directly correlated with the U-content. Areas with high U-content and highly damaged show crystalline, randomly oriented nanoparticles that are interpreted as decomposition products of the metamictization process. On the other hand, areas with lower U-content and radiation dose contained within 0.7×10 16 α-event/mg, although severely damaged, still preserve single-crystal appearance. Such areas, noticeably consisting of relicts of the original samarskite Structure, were deeply investigated by electron diffraction techniques. Surprisingly, the retrieved crystal Structure of untreated samarskite is consistent with aeschynite and not with ixiolite (or columbite), as believed so far after X-ray diffraction experiments on annealed samples. In particular, the resolved Structure is a niobioaeschynite-(Y), with Pnma space group, cell parameters a = 10.804(1), b = 7.680(1), c = 5.103(1) A, and composition (Y 0.53 Fe 0.22 Ca 0.10 U 0.09 Mn 0.07 ) Σ=1 (Nb 1.07 Ti 0.47 Fe 0.34 Ta 0.07 W 0.06 ) Σ=2 O 6 . If this finding can be confirmed and extended to the other members of the group [namely samarskite-(Yb), calciosamarskite, and ishikawaite], then the samarskite mineral group should be considered no longer as an independent mineral group but as part of the aeschynite group of minerals. It is finally suggested that the rare crystalline sub-micrometric ixiolite domains, occasionally spotted in the sample by TEM, or the nanoparticles detected in highly metamict areas interpreted as decomposition product of the metamictization process, which may have in fact the ixiolite Structure, act as seeds during annealing, leading to the detection of ixiolite peaks in the X-ray powder diffractograms.

Yves Jean - One of the best experts on this subject based on the ideXlab platform.

Yasushige Kuroda - One of the best experts on this subject based on the ideXlab platform.

Giancarlo Capitani - One of the best experts on this subject based on the ideXlab platform.

  • what is the Actual Structure of samarskite y a tem investigation of metamict samarskite from the garnet codera dike pegmatite central italian alps
    American Mineralogist, 2016
    Co-Authors: Giancarlo Capitani, Enrico Mugnaioli, Alessandro Guastoni
    Abstract:

    We investigated, by scanning and transmission electron microscopy (SEM, TEM), wavelength- and energy-dispersive spectroscopy (WDS, EDS), and electron diffraction tomography (EDT), several (Y-REE-U-Th)-(Nb-Ta-Ti) oxides from the Garnet Codera dike pegmatite (Central Italian Alps). These oxides have compositions in the samarskite-(Y) field and yield an amorphous response from the single-crystal X-ray diffractometer. Backscattered electron images reveal that the samples are zoned with major substitutions involving (U+Th) with respect to (Y+REE). At the TEM scale, the samples show a continuous range of variability both in terms of composition and in radiation damage, and the amount of radiation damage is directly correlated with the U-content. Areas with high U-content and highly damaged show crystalline, randomly oriented nanoparticles that are interpreted as decomposition products of the metamictization process. On the other hand, areas with lower U-content and radiation dose contained within 0.7×10 16 α-event/mg, although severely damaged, still preserve single-crystal appearance. Such areas, noticeably consisting of relicts of the original samarskite Structure, were deeply investigated by electron diffraction techniques. Surprisingly, the retrieved crystal Structure of untreated samarskite is consistent with aeschynite and not with ixiolite (or columbite), as believed so far after X-ray diffraction experiments on annealed samples. In particular, the resolved Structure is a niobioaeschynite-(Y), with Pnma space group, cell parameters a = 10.804(1), b = 7.680(1), c = 5.103(1) A, and composition (Y 0.53 Fe 0.22 Ca 0.10 U 0.09 Mn 0.07 ) Σ=1 (Nb 1.07 Ti 0.47 Fe 0.34 Ta 0.07 W 0.06 ) Σ=2 O 6 . If this finding can be confirmed and extended to the other members of the group [namely samarskite-(Yb), calciosamarskite, and ishikawaite], then the samarskite mineral group should be considered no longer as an independent mineral group but as part of the aeschynite group of minerals. It is finally suggested that the rare crystalline sub-micrometric ixiolite domains, occasionally spotted in the sample by TEM, or the nanoparticles detected in highly metamict areas interpreted as decomposition product of the metamictization process, which may have in fact the ixiolite Structure, act as seeds during annealing, leading to the detection of ixiolite peaks in the X-ray powder diffractograms.

Sha Jing - One of the best experts on this subject based on the ideXlab platform.

  • Actual Structure, thermodynamic driving force, and mechanism of benzofuranone-typical compounds as antioxidants in solution.
    The journal of physical chemistry. B, 2011
    Co-Authors: Xiao-qing Zhu, Jian Zhou, Chun-hua Wang, Sha Jing
    Abstract:

    5,7-Ditert-butyl-3-(3,4-dimethylphenyl)benzofuran-2(3H)-one (HP-136) (1H) and its 30 analogues (2H−5H) as benzofuranone-typical antioxidants were synthesized. The Structures of the benzofuranones in solid and solution were examined by using experimental and theoretical methods. The results show that the dominant Structure is the lactone form rather than the enol form both in solid and solution. The thermodynamic driving forces of the 31 benzofuranone-typical compounds to release protons [ΔGPD(XH)], hydrogen atoms [ΔGHD(XH)], and electrons [Eox(XH)] and the thermodynamic driving forces of the anions (X−) of the benzofuranones to release electrons [Eox(X−)] were determined for the first time in DMSO. The ΔGHD(XH) scale of these compounds in DMSO ranges from 65.2 to 74.1 (kcal/mol) for 1H−4H and from 73.8 to 75.0 (kcal/mol) for 5H, respectively, which are all smaller than that of the most widely used commercial antioxidant BHT (2,6-ditert-butyl-4-methylphenol, 81.6 kcal/mol), suggesting that the 31 XH could ...