The fingerprint of imperial topaz from Ouro Preto region (Minas Gerais state, Brazil) based on cathodoluminescence properties and composition

Abstract

A study of the cathodoluminescence (CL) properties of imperial topaz from Ouro Preto region (Minas Gerais state, Brazil) and its relation with trace-element composition was conducted, using scanning electron microscope cathodoluminescence (SEM-CL), optical microscope cathodoluminescence (OM-CL), cathodoluminescence-spectrometry (CL-spectrometry), electron microprobe analysis (EMPA), laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) and Raman spectrometry. Each analytical technique allowed characterization of the imperial topaz fingerprint. SEM-CL panchromatic images show different crystal growth and resorption events in imperial topaz crystals. Colour CL images indicate only blue to violet emissions. The CL-spectra indicate a broad emission band with low intensity peak at ~417 nm and a broad emission band with high intensity and major peaks at 685, 698, 711 and 733 nm. The EMPA indicates high OH content, in which the OH/(OH + F) ratio ranges between 0.35–0.43 (0.72 ≤ OH ≤ 0.86 apfu). High Cu and Zn concentrations (LA-ICP-MS) were measured in the high luminescence areas of SEM-CL images, suggesting both elements as CL-activators in imperial topaz. Raman and CL-spectra indicate high Cr concentrations, corroborated by EMPA and LA-ICP-MS results. The high Cr caused strong luminescence intensities that enabled their superimposition over the OH stretching mode (~3650 cm–1) of topaz in all Raman spectra. Among trace elements, the concentrations of Ti, V, Cr, Mn, Fe, Cu, Zn, Ga and Ge provide the fingerprint of imperial topaz.

A cathodoluminescence-assisted LA-ICP-MS study of topaz from different geological settings

“…In comparison with the study carried out by Gauzzi et al (2018) on imperial topaz crystals related to the Dom Bosco mine, the same CL behavior was observed as in the samples of our study (Figs. 3A and 3B).…”

“…These textures can be related to metamorphic activity, shown by lower CL along the rims of the grain, which can imply recrystallization and removal of impurities, thus generating a homogeneous CL response. In addition, all these crystallization features, showed by the CL-panchromatic images, lead one to infer that a primary process of crystallization took place followed by recrystallization process (Gauzzi et al 2018). The sample AP, besides being just part of a rhombic pattern and showing less luminescence, displays the same luminescence features of the sample DB (see Fig.…”

“…In this region, the topaz mineralizations occur essentially in the Northeastern area of the Dom Bosco Syncline and in the Northeastern flank of the Mariana Anticline (Dorr II 1969, Gandini 1994, Morteani et al 2002, Rojas 2008, Rojas et al 2009, Gauzzi et al 2018.…”

“…Due to this very intense alteration, there is no reliable information about the primary country rock related to the imperial topaz mineralization. Additionally, in both mines, the primary mineral assemblage related to the topaz-bearing veins is composed of kaolinite, quartz, muscovite, rutile and, rarely, euclase (Gandini 1994, Morteani et al 2002, Gauzzi et al 2018.…”

“…With respect to the trace element concentrations of topaz in other studies, Ti, V, Cr, Mn, Fe, Cu, Zn, Nb, LREE, HREE, Ta and W showed relevant variations (Leroy et al 2002, Wasim et al 2011, Wu et al 2011, Breiter et al 2013, Agangi et al 2014, 2016, Gauzzi et al 2018.…”