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Physics and Chemistry of Minerals

, Volume 42, Issue 4, pp 251–260 | Cite as

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

  • Remo WidmerEmail author
  • Anna-Kathrin Malsy
  • Thomas Armbruster
Original Paper

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(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., 1.927 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.

Keywords

Spinel MgAl2O4 Gemstone Heat treatment Mg–Al order–disorder Single-crystal X-ray Raman spectroscopy Photoluminescence spectroscopy 

Notes

Acknowledgments

We are highly indebted to Mariko Nagashima (Yamaguchi University, Japan) for performing the electron microprobe analyses. We are grateful to Rosa Micaela Danisi (Bern), Martin Fisch (Bern), and Veronica D’Ippolito (Rome, Italy) for revising a preliminary version of this manuscript. Reviews by Francesco Princivalle (Trieste, Italy) and an anonymous referee are highly appreciated.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  • Remo Widmer
    • 1
    Email author
  • Anna-Kathrin Malsy
    • 2
  • Thomas Armbruster
    • 1
  1. 1.Mineralogical Crystallography, Institute of Geological SciencesUniversity of BernBernSwitzerland
  2. 2.Gübelin Gem Lab Ltd.LucerneSwitzerland

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