Polycrystals of 'imperial' topaz from Minas Gerais state, Brazil

Autor:	Leonardo Martins Graça, Teodoro Gauzzi, Gilberto Henrique Tavares Álvares da Silva, Rafael Silva Diniz
Rok vydání:	2019
Předmět:	Materials science Scanning electron microscope 020209 energy Mineralogy 02 engineering and technology Electron microprobe engineering.material 010502 geochemistry & geophysics 01 natural sciences law.invention Topaz Geophysics Optical microscope Geochemistry and Petrology law Extinction (optical mineralogy) 0202 electrical engineering electronic engineering information engineering Gemstone engineering Orthorhombic crystal system 0105 earth and related environmental sciences Electron backscatter diffraction
Zdroj:	Mineralogy and Petrology. 113:273-283
ISSN:	1438-1168 0930-0708
DOI:	10.1007/s00710-019-00659-x
Popis:	“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.
Databáze:	OpenAIRE
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