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

Widmer, Remo ; Malsy, Anna-Kathrin ; Armbruster, Thomas

In: Physics and Chemistry of Minerals, 2015, vol. 42, no. 4, p. 251-260

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    Title
    • Effects of heat treatment on red gemstone spinel: single-crystal X-ray, Raman, and photoluminescence study
    Author
    • Widmer, Remo. Mineralogical Crystallography, Institute of Geological Sciences, University of Bern, Freiestrasse 3, 3012, Bern, Switzerland
    • Malsy, Anna-Kathrin. Gübelin Gem Lab Ltd., Maihofstrasse 102, 6006, Lucerne, Switzerland
    • Armbruster, Thomas. Mineralogical Crystallography, Institute of Geological Sciences, University of Bern, Freiestrasse 3, 3012, Bern, Switzerland
    Document Type
    • Postprint
    Language
    • English
    Published in
    • Physics and Chemistry of Minerals, 2015, vol. 42, no. 4, p. 251-260. Springer Berlin Heidelberg
    Other Version
    OAI-PMH Identifier
    • oai:doc.rero.ch:331539
    Summary
    A red spinel, MgAl2O4, from Burma (Myanmar) containing as chromophores ca. 0.5wt% of each Cr2O3 and V2O3, was sequentially heated for at least 72h 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 3m toward lower values with increasing temperature and associated Mg, Al disorder: T(Mg1-x Al x )M(Al2-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., 1.927Å), 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 721cm−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 684.5nm accompanied by poorly resolved N lines: N1 (687nm), N2 (688nm), and N3 (689nm). 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 (687.5nm 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.