Effects of heat treatment on red gemstone spinel: single-crystal X-ray, Raman, and photoluminescence study

Abstract

A red spinel, MgAl₂O₄, from Burma (Myanmar) containing as chromophores ca. 0.5 wt% of each Cr₂O₃ and V₂O₃, was sequentially heated for at least 72 h at temperatures ranging from 600 °C to 1,100 °C. The untreated and quenched samples were examined with single-crystal X-ray diffraction (XRD), Raman spectroscopy and photoluminescence spectroscopy. XRD results display a linear decrease of the cell parameter a and a continuous shift of the oxygen coordinate u, u, u at 3 m toward lower values with increasing temperature and associated Mg, Al disorder: ^T(Mg_1-x Al _x )^M(Al_2-x Mg _x )O₄. The natural spinel has x = 0.157(2) and reaches x = 0.286(4) after quenching from 1,100 °C. In its natural state, M–O and T–O distances are 1.9226(2) and 1.9361(4) Å. With increasing inversion of Mg from the tetrahedrally coordinated T to the octahedrally coordinated M site, M–O distances increase at 1,100 °C to 1.9333(4) Å and T–O distances decrease to 1.9130(8) Å. The crossover temperature, at which T–O and M–O distances become equal (i.e., 1.927 Å), is found to be at 650 °C and corresponds to an inversion parameter x = 0.208(3). With increasing heat treatment, Raman spectra of quenched samples become significantly broadened and a peak characteristic for Mg, Al disorder at 721 cm⁻¹ firstly appears for a crystal quenched from 800 °C with x = 0.248(4). At room temperature, photoluminescence spectra are dominated by a strong R line at 684.5 nm accompanied by poorly resolved N lines: N1 (687 nm), N2 (688 nm), and N3 (689 nm). N lines are caused by different Mg, Al environments of Cr³⁺. With increasing inversion parameter (x), the R line decreases in intensity and the N lines become prominent leading to strongly broadened peaks with a maximum shifted toward higher wave lengths (687.5 nm at 1,100 °C). Criteria for the detection of heat treatment on gemstone spinel applicable to gemological routine examination are provided. Extrapolation of u, a, and bond lengths from heat-treated Burma spinel toward the natural crystal suggests a retrograde “closing temperature” of ca. 400 ± 100 °C at which Mg, Al disorder was frozen.