DEGEO - Departamento de Geologia

URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8

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2010 - 20192

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Assunto: search.filters.subject.Electron backscatter diffraction

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    Polycrystals of “imperial” topaz from Minas Gerais state, Brazil.
    (2019) Araújo, Teodoro Gauzzi Rodrigues de; Silva, Gilberto Henrique Tavares Álvares da; Diniz, Rafael Silva; Graça, Leonardo Martins
    “Imperial” topaz is a gemstone variety that occurs in the Ouro Preto region (Minas Gerais state, Brazil). Polygonal sectors within the core and rims of topaz crystals, were optically observed but without consensual explanations about them. With the aid of optical microscopy, scanning electron microscopy-cathodoluminescence (SEM-CL), backscattered electrons (BSE) imaging, electron probe micro-analyser (EPMA) chemical analyses and electron backscatter diffraction (EBSD) maps, the present study intended to demonstrate the distinct crystallographic orientations within “imperial” topaz and relate the polygonal sectors with the compositional data. Cross-polarised transmitted-light photomicrographs show a well delimited and optically heterogeneous central rhombic area at (21 l) in the cores, and quadrant-like and alternated extinction areas at (200, (010) and (110) in the rims. Scanning electron microscopy-cathodoluminescence images show a central rhombic area heterogeneously luminescent in the core, and dark and homogeneous rims. Grey and completely homogeneous BSE images and EPMA results corroborate constant and homogeneous major composition of “imperial” topaz. Electron backscatter diffraction maps collected in the rim region show different areas and microstructural features instead of a uniform microstructure. The respective pole figures of orthorhombic system yielded multiple (001) poles disoriented in higher than 15° from each other. These results display numerous c-axes, suggesting distinct crystallographic orientations, and no reduction in the orthorhombic symmetry. Therefore, the presumed monocrystal of “imperial” topaz actually is a polycrystal.
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    Nucleation and growth of new grains in recrystallized quartz vein : an example from banded iron formation in Iron Quadrangle, Brazil.
    (2010) Lagoeiro, Leonardo Evangelista; Barbosa, Paola Ferreira
    Intracrystalline microcracks developed in quartz single crystals deformed in greenschist metamorphic conditions. A detailed study of samples collected in tabular to lens shape quartz vein was carried out to investigate how the microcracks initiated and how the microstructures evolved with the progressive deformation. A combination of light and EBSD (electron backscatter diffraction) techniques was used to analyze the microstructures and determine the crystallographic orientation of quartz grains. The crystallographic orientations of microcracks indicate that they might have initiated parallel to the direction of one of the rhombohedral planes of the host crystals. It is suggested that new grains nucleated by rotation of broken fragments from the host grains. c-axes the of host are distributed in a small-circle close to the foliation plane while the c-axes of the new grains in microcracks are more scattered when compared with the host orientations. New grains grew with their c-axes approximately perpendicular to the shortening direction.