Application of Photoluminescence Spectroscopy in the Identification of Blue Spinel

Blue magnesium aluminum spinel is a popular variety in the jewelry market. In this study, 6 cobalt-coloured spinels and 5 iron-coloured blue spinels were selected for X-ray fluorescence spectroscopy and UV-Vis spectroscopy, and photoluminescence spectroscopy tests. The results showed that the colour of common blue spinels is mainly related to the spin forbidden transition of Fe²⁺ from ⁵E→³E, ⁵E→³T₁, ⁵E→³T₂ and the spin forbidden transition of Fe³⁺ from ⁶A_1g→ ⁴A_1g. The colour of natural cobalt spinels are mainly caused by the transition of Co²⁺ from ⁴A₂→⁴T₁, and also affected by Fe²⁺ and Fe³⁺.The photoluminescence spectra of 11 blue magnesium-aluminum spinels by 405 nm excitation light source at room temperature showed obvious 5 fluorescence peak combinations, including 675, 685, 697, 707 nm and 717 nm, which are caused by ²E_g→⁴A_2g transition of Cr³⁺. The strong zero phonon line near 685 nm and the vibron sideband of the line are the main luminescent peaks. The photoluminescence spectra tested by 532 nm excitation light source at room temperature showed that the cobalt spinel samples have a wide luminescence peak at 645 nm, which are generated by Co²⁺ from ⁴T₁(P)→⁴A₂(F). Therefore, photoluminescence spectroscopy is helpful to detect the presence of lower content of Co and Cr elements, and provides an important basis for the analysis of trace elements and variety determination of blue magnesium aluminum spinel species.