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

Abstract

ABSTRACTA 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.