SPECTROSCOPIC THERMAL VARIATION AND COLOR MECHANISM OF KUH-I-LAL PINK SPINEL FROM TAJIKISTAN

The Kuh-i-Lal spinel deposit in Tajikistan is located in the Goran metamorphic series in the southwest of the Pamir Plateau and occurs in the forsterite lens of the magnesian skarn. The spectroscopic thermal variation characteristics and color mechanism of spinel in this area were studied by using UV-Vis spectrometer, photoluminescence spectrometer, infrared spectrometer and LA-ICPMS combined with heat treatment (ranging from room temperature to 1000 ℃, constant temperature time 1 h). The results show that the chemical composition of Kuh-i-Lal spinel is relatively rich in Fe, Cr and poor in V. The mass fraction of FeO_T is 0.25-0.47wt%, while Cr is 0.016-0.082wt%, V is 0.016-0.038wt%. At room temperature, the UV-Vis spectra of these spinels are characterized by a broad absorption band at 389-416nm caused by ⁴A_2g→⁴T_1g(⁴F) of Cr³⁺ and ³T_1g(³F)→³T_1g(³P) of V³⁺, and another one at around 542nm caused by ⁴A_2g→⁴T_2g(⁴F) of Cr³⁺ and ³T₁(³F)→³T₂(³F) of V³⁺. The photoluminescence spectra of spinel show a group of emission peaks with 685nm (R-line) as the main peak caused by ²E(²G)→⁴A₂(⁴F) of Cr³⁺. There is a group of N-lines caused by lattice defects in spinel between 687-692nm. The emission intensities of R-line and N-lines are independent of the content of Cr³⁺. The results of infrared spectrum display that two strong absorption bands at 695cm^-1 (ν₁) and 533cm^-1 (ν₂) respectively are related to the octahedral lattice. The band at 581cm^-1 (ν₃), is identified as the combined complex vibration of tetrahedral and octahedral cations.

Under the condition of various temperature (ranging from room temperature to 1000 ℃), the FWHM of R-line and N-lines increases with temperature (beginning from 750 ℃). After heating to 750 ℃, the intensity of N₁ peak at 687nm increases gradually. At 800 ℃, the intensity of N₁ peak exceeds that of R-line, then cease increasing after 825 ℃. In the process of N₁ rising, there is a good linear relationship between the integral ratio of R-line and N-lines and temperature. The results of infrared spectroscopy further show that during the heating process, ν₃ gradually disappeared, while ν₂ moves toward high frequency. This change is related to the variation of M-O and T-O bond length in spinel lattice. In this research, the color mechanism of pink Kuh-i-Lal spinel and the variation characteristics of optical spectrum during heat treatment process are discussed in detail. We record the change characteristics of infrared spectrum during heating process, and explore the mechanism of spectroscopic thermal variation.