gems, corundum, inclusions.5, Sri Lanka.12

The turbid/translucent, near colorless (milky) metamorphic sapphire samples from Sri Lanka have been characterized after the heat treatment in N₂ at 1650 °C. As-received sapphire specimens became bluish-colored and exhibited more clarity after the heat treatment. It was found that the color change at inclusions zoning region is attributed by the dissolution. As-received samples contain the micro/nano inclusions such as rutile (TiO₂), ilmenite (FeTiO₃), spinel (MgAl₂O₄)/ulvospinel (Fe₂TiO₄) and apatite (Ca₅(PO₄)₃), which were dissolved by the heat treatment and form the blue color through Fe²⁺/Ti⁴⁺ charge transferring. The microstructures become different because as the dissolution of apatite (Ca₅(PO₄)₃(OH,F,Cl)) in alumino silicates (Al₂SiO₅) occurred, resulting in morphological change with the appearance of (Ca, Mg, Al) silicate on the surface. Both as-received and heat-treated samples showed the rhombohedral crystal structure of Al₂O₃.