CR3+ PHOTOLUMINESCENCE AND TRACE ELEMENT CORRELATIONS IN METAMORPHIC SAPPHIRE

Trivalent chromium in corundum is an efficient luminophore with intense red luminescence emission (the R-lines at 694 and 693 nm) excited by ultraviolet and blue to green light. Changes in luminescence intensity result from differences in Cr³⁺ concentration, as well as the presence or absence of other trace elements (TE) that quench or co-activate luminescence. When mapped throughout a crystal, photoluminescence (PL) intensity reveals internal textures. Paired with TE concentrations, these relationships provide a more complete picture of local geochemistry during sapphire crystallization, either showing changes during growth present as zoning or consistent chemistry with homogeneous internal textures. If sapphires from different geographic localities formed under distinct conditions, then the luminescence and chemical features will not be the same. Ultimately, characterizing the PL textures and intensities as well as the TE concentrations of sapphires from different deposits will show whether sapphires mined from different geographic localities can be distinguished based on spectral and geochemical features.

Correlative line transects of TE concentrations, obtained by LA-ICP-MS, and PL spectra were measured in blue sapphires from Burma, Madagascar, and Sri Lanka. Because PL mapping of large crystals is time-intensive, we designed a photomicrography technique to image whole-grain internal textures where R-line PL images can be collected in 2 minutes or less using 395 nm light illumination and a 694 nm bandpass filter. Preliminary PL and TE analyses show that luminescence variations are caused by interactions between Cr³⁺ and other TE and rarely from changes in Cr concentration alone. Even sapphire domains with the brightest PL have low Cr concentrations, usually from 50 ppmw to below detection limit, and often exhibit decreased Fe contents rather than increased Cr. Fe concentrations are typically a few hundred to thousands of ppmw in brighter PL domains, indicating that PL quenching by Fe is less effective than Cr activation. Internal zoning seen with PL is often accompanied by Mg and Ti zoning in TE traverses and less often by zoning in Fe, Ga, V, or Be. When zoned, trends in Mg and Ti concentration are similar to each other but distinct from trends in Ga and Fe or V and Fe, which also tend to have similar patterns.