New Evidence for Low-Temperature Heat Treatment Detection in Vietnamese Magmatic Sapphires Using FTIR Spectroscopy

Low-temperature heat treatment in magmatic sapphires can be difficult to detect because conventional indicators, including hydroxyl-related absorptions and short-wave ultraviolet fluorescence, are often weak, absent, or inconclusive in Fe-rich material. This study evaluates Fourier transform infrared (FTIR) reflectance spectroscopy as a method for detecting low-temperature heat treatment in Vietnamese magmatic sapphires. A total of 38 natural sapphires from basalt-related deposits in southern Vietnam, comprising 23 samples from Dak Lak and 15 samples from Phan Thiet, were analyzed before and after heating at 1100 °C for 3 h under oxidizing conditions. Particular attention was given to spectral modifications within the fingerprint region (1000–500 cm−1), which is dominated by lattice-vibrational modes of α-Al2O3. Following heat treatment, all samples exhibited a systematic shift in the dominant reflectance trough-center position from 646.4 ± 2.0 cm−1 to 630.2 ± 0.8 cm−1 together with a substantial increase in trough depth from 0.052 ± 0.025 to 0.705 ± 0.118%R (p < 0.0001). The spectral feature also became narrower and more sharply defined after heating, consistent with modification of the local vibrational environment within the corundum lattice. In contrast, hydroxyl-related absorptions in the 3100–3700 cm−1 region were weak, variably developed, or absent and showed no reproducible treatment-related behavior. Complete separation between untreated and heat-treated samples was achieved when trough-center position and trough depth were evaluated jointly. These results demonstrate that FTIR fingerprint-region parameters derived from intrinsic α-Al2O3 lattice vibrations provide robust indicators of low-temperature heat treatment in Vietnamese magmatic sapphires and may serve as complementary criteria when conventional OH-based indicators are inconclusive.