NEW DATA OF RS PHOTOLUMINESCENCE AND LA-ICP-MS ELEMENT MAPPINGS ON CR3+ BEHAVIOR AT THE CONCENTRATIONS < 100 > 0 PPMW WITHIN THE α-AL2O3 LATTICE: CASE STUDY OF ILMEN BLUE SAPPHIRES, RUSSIA

The luminescence features of α-Al₂O₃ (mineral corundum) doped by Cr³⁺ ions (α-Al₂O₃:Cr³⁺) have been studied since the time of ruby solid-state lasers development. In the α-Al₂O₃:Cr³⁺ luminescence spectra induced by the Raman laser-emission (PL RS), the most intense peaks at around 692 nm (R2-line) and 694 nm (R1-line) are due to Cr³⁺ single ions accompanied by sidebands centered at around 710 and 714 nm, and less-intense apparent maxima at ~701 and ~705 nm are due to Cr³⁺ pairs (N-lines). At the Cr content > 100 - 300 ppmw, when the N-lines appear in α-Al₂O₃:Cr³⁺ RS PL spectra, their intensities ratio toward those of sidebands was applied for Cr³⁺ quantitative calculations in the corundum lattice (Häger & Dung, Eur. J. Mineral., 2000; Sorokina et al., Amin, 2016). Meanwhile the use of R-lines intensities for the estimation of Cr³⁺ relative concentration was eliminated due to their quenching in the presence of iron (Nasdala et al., EMU notes in Mineral., 2003).

Corundum syenite pegmatites in the Ilmen Mountains belonging to the south part of Uralian orogenic belt, present the source of transparent blue-colored sapphire megacrysts measuring up to 6 cm wide (Sorokina et al., G&G, 2016). RS Cr³⁺ PL mapping of R1-line peak area demonstrated oscillatory behavior with at least 2 well-defined zones. Laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) element mapping of Fe, V and Ga showed the minor degree of element heterogeneity within the α-Al₂O₃; Cr content varied from about 1 to 60 ppmw and its chemical map was found to be consistent with the oscillatory zonation of RS Cr³⁺ PL map in R1-line peak area.

Thus, undertaken research showed:

1. RS Cr³⁺ PL mapping of R1-line peak area may be potentially used for relative Cr³⁺ calculations at content < 100 > 0 ppmw within α-Al₂O₃;

2. The source of chromium within Ilmen blue sapphires may not be linked to the marine sediments as an evidence of no correlation observed among Cr, Fe, V, and Ga traces on LA-ICP-MS maps. This geochemical phenomenon provides the clues on the genetic nature of these gem corundums, which is different from those originated by continent-to-continent collision.

We are thankful to Dr. Wuyi Wang (GIA New York) and Dr. Sebastien Meffre (University of Tasmania, Australia) for their assistance in development of RS and LA-ICP-MS mapping techniques, respectively.