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Carmela Vaccaro - One of the best experts on this subject based on the ideXlab platform.

  • Crystal chemistry and petrologic significance of Fe3+-rich phlogopite from the Tapira carbonatite complex, Brazil
    American Mineralogist, 1996
    Co-Authors: Maria Franca Brigatti, Luca Medici, Emilio Saccani, Carmela Vaccaro
    Abstract:

    This contribution deals with the crystal chemistry of phlogopite and Fe3+-rich phlogopite from the Tapira alkaline-carbonatite complex (Brazil) to assess the petrological significance and genetic conditions of these rocks. The Tapira complex consists of a layered intrusion composed mainly of ultramafic rocks (dunite, wehrlite, clinopyroxenite, bebedourite, garnet-magnetitite, perovskite-magnetitite, and glimmerite) with subordinate carbonatite. The wide range of textural, optical, and crystal-chemical characteristics of ph logopite is related to the variation of 102' aH20,and aC02as well as magma bulk-chemical composition during fractional crystallization. Phlogopite from alkaline-silicate rocks (ranging from dunite to bebedourite) is characterized by fairly constant Al content, moderate [4]Fe3+substitution, and variable amounts of Ti. The [4]Fe3+ substitution, accompanied by crystals showing reverse Pleochroism, increases during fractional crystallization. These features correspond to crystallization at low pressure and high 102 and aH20 in the presence of moderate saturation in Ti-bearing phases, Al203 in the magma, or both. Phlogopite from silicate-carbonatite rocks, classified as ferriphlogopite on the basis of strong reverse Pleochroism related to [4]Fe3+tetrahedral substitution, also presents low to very low AI, Fe2+, and Ti contents. These features suggest very high 102' H20' and C02 conditions in the presence of strong saturation in Ti-bearing phases as well as very low Al203 content in the liquid. The crystal-structure refinements of Tapira phlogopite show that Fe3+ substitutes for Si in tetrahedral sites; Fe distribution is completely disordered, so the resulting space group is C21m. 'The octahedral-site composition is similar to that of phlogopite, the octahedral sites being preferentially occupied by Mg. The presence of Fe3+ in the tetrahedral sheet enlarges the whole structure. This enlargement is reflected by an increase in cell-edge lengths and a decrease in ,B-angle values. The increase in distortion of the tetrahedral ring (a angle up to "" 11°)is necessary for the tetrahedral and octahedral sheets to fit together.

Jianbei Qiu - One of the best experts on this subject based on the ideXlab platform.

  • Achieving high thermal stability of different rare-earth ions in a single matrix host via the manipulation of the local structure by a solid solution
    Physical chemistry chemical physics : PCCP, 2020
    Co-Authors: Xiuxia Yang, Lei Zhao, Zhichao Liu, Shuyu Tian, Hao Zhang, Jianbei Qiu
    Abstract:

    Thermal quenching seriously restricts the practical application of phosphors, particularly under high temperature and long-term working conditions. Here, we demonstrate that the as-obtained series of solid solutions of Ca2−xYxAl2Si1−xAlxO7:Tb3+ (x = 0–1, Ca2Al2SiO7 → CaYAl3O7) phosphors exhibit an adjustable optical performance, where CaYAl3O7:Tb3+ exhibits a greatly improved thermal stability with a shortened bond distance of the related polyhedron compared with Ca2Al2SiO7:Tb3+. The shrunken bond distance strengthens the pressure of the local structure and suppresses the non-radiative transition effectively, contributing to the prevention of the thermal degradation. The formed phosphor with excellent structural stability could be effectively incorporated with various lanthanide ions (Eu3+, Tb3+, Sm3+, Dy3+, and Pr3+) to address a Pleochroism output.

Boris Chauviré - One of the best experts on this subject based on the ideXlab platform.

  • Reverse dichromatism in gem andalusite related to total Pleochroism of the Fe2+-Ti4+ IVCT.
    Spectrochimica acta. Part A Molecular and biomolecular spectroscopy, 2018
    Co-Authors: Benjamin Rondeau, Sarah Chamard-bois, Emmanuel Fritsch, Franck Notari, Boris Chauviré
    Abstract:

    Abstract We document a dichromatism effect in gem andalusite that shifts from light brownish pink at low thickness to medium green at high thickness. This is roughly reverse to that usually observed in other dichromatic materials. We show that this is due to a very strong Pleochroism in andalusite, when dichromatism in other materials is due to an alexandrite effect. Here, colors varies from a strongly absorbing orangey-brown in the c direction to much less absorbing medium green in a and b directions. The brown absorption is so strong that it becomes opaque (total absorption) for thickness above about 1.5 mm (this value may vary according to concentration of the chromophore and intensity of the light source), and hence does not contribute to coloration anymore. For such high thicknesses, only the a and b directions remain transparent and hence only the green component contributes to the color: the sample appears green. For lower thicknesses, the green color combines with the orangey-brown color that becomes transparent along the c direction, resulting in a light brownish pink. From trace elements analysis in zoned samples, we also confirm that the dark orangey-brown color is due to Fe2+-Ti4+, and bring evidences that the light green color is mostly due to Fe, either in the form of isolated Fe3+ or Fe2+-Fe3+ IVCT, with possibly some contributions from isolated Mn3+.

Hideo Ishizuka - One of the best experts on this subject based on the ideXlab platform.

  • Pumpellyite from zeolite facies metabasites of the Horokanai ophiolite in the Kamuikotan zone, Hokkaido, Japan
    Contributions to Mineralogy and Petrology, 1991
    Co-Authors: Hideo Ishizuka
    Abstract:

    Pumpellyite occures in zeolite facies metabasites of the Horokanai ophiolite in the Kamuikotan zone, Hokkaido, Japan, filling veins or amygdules, replacing igneous plagioclase or clinopyroxene or olivine, and occupying the matrix. Its composition and Pleochroism vary greatly even within a single sample, but appear to be related to its mode of occurrence. Thus, the most Al-rich pumpellyite with pale green to green Pleochroism develops in pseudomorphs after plagioclase, whereas the most Fe^*-rich variety with deep green to brown Pleochroism occurs in the matrix. In low-grade metamorphic rocks which commonly contain relict minerals, chemical equilibrium is attained only locally. This results in the correlation of the composition of pumpellyite with its mode of occurrence, such as the precursor phases which are replaced by pumpellyite. On the other hand, among pumpellyites occurring in similar mode and coexisting with Ca-zeolite (laumontite or wairakite), epidote, chlorite and quartz, the Al content tends to be enriched in the wairakite-bearing metabasites over the laumontite-bearing metabasites. It follows that the composition of pumpellyite is also dependent upon the temperature of metamorphism.

Jogirdas Vainauskas - One of the best experts on this subject based on the ideXlab platform.

  • Halogen bonding to the azulene π-system: cocrystal design of Pleochroism.
    Chemical communications (Cambridge England), 2020
    Co-Authors: Jogirdas Vainauskas, Oleksandr S. Bushuyev, Christopher J. Barrett, Filip Topić, Tomislav Friščić
    Abstract:

    We describe the rational development of a design for pleochroic molecular solids, by using C-Iπ halogen bonds to pre-organise different chromophores in a cocrystal. Using the structure of a known naphthalene cocrystal as a blueprint, we demonstrate how a highly robust C-Iπ motif permits the systematic exchange of original cocrystal components with azobenzene and azulene, resulting in optically interesting dichroic or pleochroic materials.

  • Halogen Bonding to the Azulene Pi-System: Cocrystal Design of Pleochroism
    2020
    Co-Authors: Jogirdas Vainauskas, Filip Topic, Oleksandr S. Bushuyev, Christopher J. Barrett, Tomislav Friscic
    Abstract:

    We describe the rational development of a design to create pleochroic molecular solids, by using C-I...pi halogen bonds to pre-organise different types of chromophores in a cocrystal. Using as a blueprint the structure of a non-dichroic cocrystal of naphthalene, we show how the systematic introduction of chromophores able to act as halogen bond donors or acceptors enables the construction of optically interesting dichroic or pleochroic solids, demonstrating the reliability of C-I...pi halogen bonds in designing new properties.<br>