Relation between cathodoluminescence and trace-element distribution of magmatic topaz from the Ary-Bulak massif, Russia

Abstract

In order to define the cathodoluminescence (CL) properties of magmatic topaz and its relation with trace-element composition, we studied topaz phenocrysts from the Ary-Bulak ongonite massif, Russia using a wide array of analytical techniques. Scanning electron microscopy CL panchromatic images reveal strong variations, which define micrometre-scale euhedral growth textures. Several truncations of these growth textures occur in single grains implying multiple growth and resorption events. The CL-spectra of both CLbright and -dark domains have a major peak in the near-ultraviolet centred at 393 nm. Cathodoluminescence images taken after several minutes of electron bombardment show decreasing emission intensity. Electron microprobe analyses indicate high F concentrations (average OH/(OH + F) = 0.04 calculated by difference, 100 wt.% – total from electron probe microanalyses), consistent with what has been found previously in topaz-bearing granites, and the OH stretching vibration (∼3653 cm–1) was detected in Raman spectra. Laser ablation inductively-coupled plasma mass spectrometry traverses performed across the CL textures detected trace elements at ppm to thousands of ppm levels, including: Fe, Mn, Li, Be, B, P, Nb, Ta, W, Ti, Ga, light rare-earth elements, Th and U. Lithium, W, Nb and Ta appear to be correlated with CL intensity, suggesting a role for some of these elements in the activation of CL in topaz. In contrast, no clear correlation was found between CL intensity and F contents, despite the fact that the replacement of OH for F is known to affect the cell parameters of topaz.

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

“…These peaks have the following assignments: symmetric Si–O ring deformation (243 cm –1 ; Skvortsova et al , 2013); various vibrational modes of [SiO 4 ]-tetrahedra (at 272, 291, 554, 841 and 916 cm –1 ; Beny and Piriou, 1987; Downs, 2006; Skvortsova et al , 2013); stretching and bending modes of [AlO 6 ]-octahedra coupled with the bending modes of [SiO 4 ]-tetrahedra (at 364, 405 and 463 cm –1 ; Beny and Piriou, 1987; Skvortsova et al , 2013); stretching modes of Al–F (at 336 cm –1 ; Agangi et al 2016); in-plane bending OH-groups (at 1160 cm –1 ; Prasad and Gowd, 2003). Even after being observed in other studies, such as Skvortsova et al (2013) and Agangi et al (2016), the interpretation of peaks at 519 and 642 cm –1 remains unclear. In our study, there was not a clear relation between the CL-bright zones and the Raman spectra.…”

“…This spectrum was acquired in a section perpendicular to the c axis. The peak assignments: ‘a’ symmetric Si–O ring deformation; ‘b’ vibrational modes of [SiO 4 ] tetrahedra; ‘c’ stretching modes of [AlO 6 ] octahedra; ‘d’ stretching modes of Al–F; ‘e’ in-plane bending OH-groups; ‘u’ non assigned peaks (Beny and Piriou, 1987; Downs, 2006; Skvortsova et al , 2013; Agangi et al , 2016). …”

“…For example, those from S- and A-type granites from the Krušné Hory/Erzgebirge area in Czech Republic/Germany, have a OH/(OH + F) of ~ 0.05 (Breiter et al , 2013). Topaz from F-rich ongonites of the Ary-Bulak massif, Russia, have a OH/(OH + F) of ≤ 0.09 (Agangi et al , 2014, 2016).…”

“…However, the studies carried out by Zhang et al (2002) with topaz crystals of high- to ultrahigh-pressure metamorphic rocks (kyanite topaz-bearing quartzites) from Sulu terrane (Eastern China), have showed 0.35 ≤ X OH ≤ 0.55. With respect to the trace-element composition of topaz, other studies show significant variations in Ti, V, Cr, Mn, Fe, Cu, Zn, Ga and Ge (Leroy et al , 2002; Wasim et al , 2011; Wu et al , 2011; Breiter et al , 2013; Agangi et al , 2014, 2016).…”

“…Topaz is a F/OH-bearing nesosilicate, Al 2 SiO 4 (F,OH) 2 , that is usually found as an accessory and late-stage crystallization mineral in a wide variety of geological settings, such as F-rich felsic magmatic (rhyolites and granites) rocks, late-magmatic rocks (pegmatites) and post-magmatic rocks (greisens), hydrothermal veins and also in high-temperature and high- to ultrahigh-pressure metamorphic rocks, especially kyanite topaz-bearing quartzites (Zhang et al , 2002; Marshall and Walton, 2007; Agangi et al , 2014, 2016).…”