Chemical Characteristic and changes after Beryllium heat-treatment of Blue Sapphire from Sri Lanka

Abstract

A total of 92 natural sapphire samples from Sri Lanka underwent heat treatment with beryllium (Be) and were studied using standard and spectroscopic instruments. They were categorized into five groups based on color and transparency: light blue, medium blue, light yellow, milky white, and violet-purple. The objective of this study is to investigate the impact of heat treatment involving beryllium on the sapphires, with a specific focus on achieving a desirable blue coloration. All groups of sapphire samples were subjected to heat treatment using gas, electric, and oil furnaces at various temperatures and in both oxidizing and reducing atmospheres. These methods were based on successful practices by a professional heat treater named Mr. Thawatchai Somjaineuk from Chantaburi. The results obtained from all the heating experiments conducted in this study indicate that the medium blue and violet-purple groups exhibit the strongest saturation and tone among the sapphire samples that developed a blue color after heating, showing the most significant improvement in color. The gemological properties of the sapphires remained consistent before and after heat treatment, aligning with typical characteristics found in sapphires worldwide. Orange luminescence, attributed to a Mg-related defect or color center, was observed in Sri Lankan sapphires. Beryllium heat treatment resulted in stronger orange fluorescence, potentially due to an increased concentration of defect centers induced by beryllium. The treatment also caused noticeable changes in inclusion characteristics, including the rounding of partially healed fissures, shiny fissures surrounding included crystals, and the breakdown of long needles into fine dot-like structures, leading to improved clarity.

Chemical analysis revealed high concentrations of Fe, significant amounts of Ti and Mg, and measurable levels of Ga, V, and Cr in the sapphires. The concentrations of trace elements remained consistent before and after heat treatment, except for the presence of Be, ranging from 11 to 26 ppma after treatment. XANES analysis showed that Fe predominantly existed as Fe3+ substituting Al3+ in the octahedral site, and Ti as Ti4+ substituting Al3+, regardless of the specific heating conditions. The infrared absorption spectra of the sapphires underwent changes during the three-step heat treatment process. Unheated yellow sapphires exhibited absorption bands associated with Mg-related OH-stretching and trapped-hole defects. Heating resulted in the removal of the 3309 cm-1 band in most samples and the appearance of broad absorption bands at approximately 2493 and 3055 cm-1, consistent with characteristics of Be-diffusion treated corundum. These absorption bands were mostly eliminated in all samples after the final heating step, although a few samples still exhibited bands at 2628 and 3055 cm-1. The presence of these infrared absorption bands can serve as an indication of blue sapphires that have undergone heat treatment with beryllium.

The combination of color appearance, absorption spectra, and chemical data suggests the role of beryllium in the coloration of sapphires. Be-trapped hole yellow color centers formed during oxidation heating can be deactivated through reduction heating. The resulting color is determined by the Mg to Ti ratio, with no influence from Be. However, most beryllium-treated blue sapphires had an Mg/Ti ratio greater than 1. The presence of higher energy level elements, such as silicon (Si), may facilitate the formation of blue color by allowing remaining titanium to pair with iron. Therefore, the blue coloration is mainly caused by strong broad absorption bands of Fe2+/Ti4+ intervalence charge transfer (IVCT) mechanism without Fe2+/Fe3+ IVCT. The study highlights the complex interplay of elements within the crystal lattice and contributes to our understanding of color development mechanisms in beryllium-treated sapphires.