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Mineralogy and Petrology

, Volume 113, Issue 3, pp 273–283 | Cite as

Polycrystals of “imperial” topaz from Minas Gerais state, Brazil

  • Teodoro GauzziEmail author
  • Gilberto Álvares da Silva
  • Rafael Silva Diniz
  • Leonardo Martins Graça
Original Paper
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Abstract

“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.

Keywords

“Imperial” topaz Polygonal sectors Polycrystals Crystallographic orientation Electron backscatter diffraction 

Notes

Acknowledgements

The authors would like to thank Microanalysis Laboratory, a member of the Microscopy and Microanalysis Network of Minas Gerais State/Brazil/FAPEMIG, for performing optical microscopy and SEM-CL imaging and EPMA analyses, and NanoLab of the Department of Metallurgical and Materials Engineering for obtaining BSE images and EBSD analyses. The authors also wish to thank Ricardo Scholz (Universidade Federal de Ouro Preto) for very helpful comments and literature suggestions. Constructive reviews by Daniel Atencio, Herta Effenberger and an anonymous expert, and constructive comments by journal editor Anton Beran, are greatly appreciated.

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Copyright information

© Springer-Verlag GmbH Austria, part of Springer Nature 2019

Authors and Affiliations

  • Teodoro Gauzzi
    • 1
    Email author
  • Gilberto Álvares da Silva
    • 2
  • Rafael Silva Diniz
    • 1
  • Leonardo Martins Graça
    • 1
  1. 1.Department of GeologyUniversidade Federal de Ouro PretoOuro PretoBrazil
  2. 2.Department of Metallurgical and Materials EngineeringUniversidade Federal de Ouro PretoOuro PretoBrazil

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