SPECTROSCOPIC CHARACTERISTICS OF HEATED BLUE/VIOLET SPINELS: UV-VIS-NIR AND PHOTOLUMINESCENCE STUDY

Blue spinel, particularly vibrant cobalt blue color, is another important variety in addition to the most popular Cr-containing pink to red colors. Heat treatment with Co diffusion may apply to improve visual appearance of blue spinel in previous reports, however, it is unknown what effect heat treatment may have on blue spinels in the absence of diffusion of external chemical components. The dominant chromophores in blue/violet spinels include both tetrahedral (T) and octahedral (M) Fe²⁺, Fe³⁺ as well as Co²⁺ at T site. This study involved exploratory heat treatment of blue/violet spinels from various localities (Madagascar, Myanmar, Tanzania, and Vietnam) to investigate the change of chromophores caused by heat treatment in oxidizing condition. The samples were separated into three groups: violet spinel and both Fe- and Co-dominated blue spinels. The effects of heat treatment were monitored before and after heating using UV-Vis-NIR and photoluminescence (PL) spectroscopy as well as carefully color-calibrated photography. Progressively heating in the range of 600-1000°C for 4 hours showed that the treatment does not improve color appearance. There was no significant change in violet colored spinel. Blue coloration became more grayish after heating at 800-900°C. The color appearance in Fe-dominated blue spinels changed more dramatically than for Co-dominated blue spinels. UV-Vis-NIR spectra of unheated blue/violet spinels showed relatively similar absorption spectra with different Fe concentrations and ratio of Fe/Co. They consist of a strong UV-edge absorption at approximately below 330 nm, multiple bands between 350 and 1100 nm of Fe²⁺, Fe³⁺ at T and M sites, and also multiple bands between 500 and 670 nm of Co²⁺ at T site. After heat treatment, the spectra showed broader absorption bands in general with less prominent bands below 650 nm and more prominent bands at approximately 920 nm. PL spectra of Cr emission peaks in heated materials were broaden after heated at 800-900 °C. The change in visual appearance and spectroscopic data supported the disorder of Mg and Al ions in spinel structure and the changes of valence and site distribution of Fe chromophores. Given the less desirable color in treated blue spinels in this preliminary study, this suggests that any heated blue/violet spinels were likely unintentionally treated.