Color Mechanism and Spectroscopic Thermal Variation of Pink Spinel Reportedly from Kuh-i-Lal, Tajikistan

The color mechanism of pink spinel from Kuh-i-Lal in the Pamir Mountains of Tajikistan was studied using photo luminescence, mid-infrared, and ultraviolet/visible spectroscopy. This included studying the variations in spectra after heating to a series of temperatures ranging from 300° to 1000°C. Laser ablation–inductively coupled plasma–mass spectrometry was used to measure the trace elements present. The results reveal that the color is caused by the combined effect of spin-allowed transitions of Cr 3+ , Fe 3+ , V 3+ , and Fe 2+ -Fe 3+ intervalence charge transfer mechanisms. The photoluminescence spectra show that during heat treatment, the intensity of the N 1 peak (687 nm) and the full width half maximum of the R-line and N-lines increase with temperature. Following heating in the range of 750°–825°C, there is a linear relationship between temperature and the ratio of integral areas of the R-line and N-lines. The mid-infrared spectra indicate that the band at 581 cm –1 ( ν 3 ) gradually disappears during the heating process, which is likely related to the variation of octahedral M-O and tetrahedral T-O bond length in the spinel lattice.