gems, spinel, spinel treatments, heat treatment, Raman spectroscopy

A red spinel, MgAl₂O₄, from Burma (Myanmar) containing approximately 0.5 wt% each of Cr₂O₃ and V₂O₃ as chromophores, was sequentially heated for at least 72 h at temperatures ranging from 600 °C to 1100 °C. The untreated and quenched samples were analyzed using single-crystal X-ray diffraction (XRD), Raman spectroscopy, and photoluminescence spectroscopy. XRD results show a linear decrease in the cell parameter a and a continuous shift of the oxygen coordinate (u, u, u) at 3̅m toward lower values with increasing temperature and associated Mg–Al disorder, expressed as T(Mg₁₋ₓAlₓ)M(Al₂₋ₓMgₓ)O₄. The natural spinel has an inversion parameter x = 0.157(2), which increases to x = 0.286(4) after quenching from 1100 °C. In its natural state, the M–O and T–O bond distances are 1.9226(2) Å and 1.9361(4) Å, respectively. As Mg inverts from the tetrahedral (T) to the octahedral (M) site, M–O distances increase to 1.9333(4) Å and T–O distances decrease to 1.9130(8) Å at 1100 °C. The crossover point, where T–O and M–O distances equalize at ~1.927 Å, occurs at ~650 °C, corresponding to x = 0.208(3). With increasing temperature, Raman spectra of quenched samples broaden notably, and a peak at 721 cm⁻¹—indicative of Mg–Al disorder—first appears in the sample quenched from 800 °C (x = 0.248(4)). At room temperature, photoluminescence spectra are dominated by a strong R line at 684.5 nm, along with poorly resolved N lines (N₁ at 687 nm, N₂ at 688 nm, N₃ at 689 nm), attributed to variations in the Mg–Al coordination environment around Cr³⁺. As the inversion parameter increases, the R line weakens and the N lines intensify, causing peak broadening and a wavelength shift to 687.5 nm at 1100 °C. Criteria are proposed for identifying heat treatment in spinel during routine gemological analysis. Extrapolation of oxygen positional parameter u, lattice parameter a, and bond lengths from the heat-treated samples back to the natural state suggests a retrograde “closing temperature” of approximately 400 ± 100 °C, below which Mg–Al disorder becomes fixed.