Analysis and Identification of Cobalt Lattice Diffused Vietnam Spinel by Using UV-Vis-NIR Spectroscopy

The yield of blue spinel from Vietnam is high, while the spinel with excellent color and good quality is rare. Therefore, it is necessary to improve the color and the aesthetics for some blue spinel. Gray-blue spinel samples from the Luc Yen district Vietnam are selected for composition tests, according to the analysis of X-ray fluoresce (XRF) spectrum and laser-induced breakdown spectroscopy (LIBS). The spinel originated from Vietnam is magnesia-alumina spinel containing trace amount of Fe and Zn. By ultraviolet-visible-near infrared spectrum (UV-Vis-NIR), the samples have 7 absorption bands which approximately located at 371, 459, 478, 548, 580, 630, and 740 nm in the range of 230-800 nm. Besides, the positions of the absorption bands are relatively fixed, indicating that spin forbidden transitions between the Fe²⁺ and Fe³⁺ is the main reason for the color of natural blue spinel. The charge-coupled effect plays an important role in the color-change at the same time. Samples are compared before and after the cobalt lattice diffusion at the stable temperature of 1 400℃ for 24 h, and the treated samples are found to have stronger wide absorption bands located around 460 nm and 550-700 nm, because the cobalt ions pass into spinel's crystal structure. Co³⁺ and Fe³⁺ enter into the tetrahedron structure from the octahedron structure and then interact with adjacent Co²⁺ and Fe²⁺, which makes the color of the spinel's tone darker and the saturation more vivid. Through the analysis of scanning electron microscopy with energy dispersive spectrometer (SEM-EDS), the depth of diffusion is about 10 μm. The immersion method is effective for detecting the gray-blue spinel treated by lattice diffusion. The color district of samples after the lattice diffusion treatment gathers in surface crevice, and the color enriches in the pits too. Moreover, the color distribution is uneven and the color is unnatural.