Quantitative Analysis of Photoluminescence Spectrum of Red Spinel from Four Localities and Its Application in Provenance Tracing

Gem-quality spinel is an important category of gemstones. Currently, deposits such as Mansin, Namya, and Mahenge, known for yielding high-value single spinel crystals, have appeared. Over the past five years, there has been over 200% increase in the number of papers related to spinel compared to the previous five years. Current research mainly focuses on using geochemical analysis and inclusions characteristics for provenance tracing; however, there are shortcomings such as overlapping discriminant features and the absence of inclusions in gem-quality spinel. Therefore, delving into the crystallographic properties and its application in rapid non-destructive testing of spinel are conducive to the developing provenance tracing technologies suitable for the domestic quality inspection industry, which is of great significant for the domestic gemmological research and industry development.

PL spectrum of spinel is constituted by a series of N-lines, each of which is associated with the arrangement of PL centers of Mg²⁺ and Al³⁺ in spinel. This laid a solid foundation for the application of PL spectrum of spinel. Based on the ordered-disordered phase transition characteristics of spinel, our research group has demonstrated in preliminary studies that the ratio of the R peak (ordered peak) to the N peak (disordered peak) in PL spectrum of spinel can finely reflect the degree of disorder, and established a thermodynamic model to quantitatively calculate its thermal history (closure temperature). In this study, 308 red spinel samples from Myanmar, Tanzania, Vietnam, and Tajikistan (Fig. 1) were studied. The main results are as follows: (1) Validation of method applicability. This study has demonstrated that even when chromium content of spinel is below the detection limit of LA-ICP-MS, and may also detect the PL spectrum of Cr³⁺ within the spinel (Fig. 2). This indicates extremely high PL efficiency of Cr in spinel, proving that our research method is not only applicable to red spinels but can also be broadly applied to the majority of gem-quality spinels. (2) Based on quantitative and qualitative analysis methods, the photoluminescence spectral characteristics of chromium ions in spinels from each locality were summarized, selecting spectral features significant for provenance identification. Under liquid nitrogen test conditions, spinels from all four localities exhibited four common N peaks of N₁, n₃, N₂, and n₅, within the zero-phonon line range (14 480-14 640 cm^-1) (Fig. 3). Meanwhile, Tanzanian spinels could be further subdivided into types Ⅰ and Ⅱ based on the feature of N₂ peak. (3) Quantitative calculations of the ordered-disordered phase transition equilibrium constants (k values) and closure temperatures of spinel samples from four localities were applied to determine discriminant for provenance tracing. The k values of Tanzanian spinels range from 1.13×10^-6-2.47×10^-4; Tajikistan spinels range from 1.75×10^-5-2.85×10^-4; Vietnam spinels range from 1.29×10^-5~3.13×10^-3; Myanmar spinels range from 1.03×10^-4-2.65×10^-3. The results indicate that the degree of disorder of spinel crystals from four localities is Myanmar/Vietnam > Tajikistan > Tanzania. Moreover. Combining with statistical methods get the closure temperature ranges of spinels from four localities. Tanzanian spinels range from 1 057 K to 1 072 K; Myanmar spinels are from 1 196 K to 1 218 K; Tajikistan spinels are from 1 128 K to 1 141 K; Vietnam spinels are from 1 057 K to 1 250 K.

In summary, this study has summarized equilibrium constants for ordered-disordered phase transitions, closure temperatures, and spectral features with provenance tracing significance based on PL spectrum of ions in spinel. We have not only accumulated amount of reliable data but also further optimized data analysis methods in quantitative calculation and statistics. These achievements have validated the practical application feasibility of our preliminary theoretical research and advanced the development of spinel provenance tracing technology. Meanwhile, this also advanced the development of the researches in using spinel as a medium to constrain geological thermal history.