corundum, Madagascar.12, inclusions.5, treatments.6, gems, zircon

Zircon inclusions in sapphires from Madagascar were examined to assess the effects of heat treatment on their gemological and spectroscopic characteristics, revealing a systematic alteration sequence between 1400 °C and 1850 °C. In unheated sapphires, the zircon inclusions appeared as transparent, slightly elongated euhedral crystals with sharp interfaces and were typically confined under high internal pressures (up to 27 kbar), often showing signs of metamictization due to natural radiation damage. At temperatures as low as 1400 °C, subsolidus reactions began in zircons with preexisting radiation damage, leading to decomposition into baddeleyite (ZrO₂) and a SiO₂-rich phase. Above 1600 °C, zircon melting and dissolution of adjacent sapphire occurred, producing baddeleyite and an Al₂O₃–SiO₂-rich quenched glass. These observations define a progressive transformation and destruction sequence of zircon inclusions with rising temperature, which can serve both as a gemological tool to detect heat treatment in zircon-bearing corundum and as a means of estimating the treatment temperature.