Analysis of UV-Vis Spectrum on Pink-Purple Spinel from Burma before and after Heat Treatment

In this paper, 5 pieces of pink-purple spinels from Burma were selected as experimental samples and heated at 1 200℃ and 1 600℃ for 10 hours respectively.Through laser induced breakdown spectroscopy (LIBS) analysis for the chemical compositions and UV-Vis spectroscopy, it was found that all samples were MgAl₂O₄ with trace amount of element Fe and the UV-Vis absorption bands were basically the same in the range 350-600 nm.The causes for their colours may be related to spin-forbidden transition of Fe³⁺ in octahedral site and Fe²⁺ in tetrahedral site and charge transfer between these pairs.Comparing UV-Vis spectra before and after heat treatment, it was found that the absorption intensity of all samples were enhaced dramatically in 350-430 nm and 500-600 nm areas.The absorption band at 350-430 nm is believed to be caused by increasing intervalence charge transfer between O^2-→Fe²⁺、Fe³⁺ after the disorder structural transition.The absorption band at 500-600 nm is believed to be caused by Fe³⁺ in tetrahedral site effected by Fe²⁺ in adjacent site.After the heat treatment at temperature of 1 600℃, the absorption intensity in near ultraviolet region increased again and led to a clear change in UV-Vis spectrum, which making the apparent colour of samples become redder, however, some samples fail to follow this result due to internal fissures and lower transparency.