Effects of heat treatment on red gemstone spinel: single-crystal X-ray, Raman, and photoluminescence study
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- 作者：Remo Widmer ; Anna-Kathrin Malsy ; Thomas Armbruster
- 关键词：Spinel ; MgAl2O4 ; Gemstone ; Heat treatment ; Mg–Al order–disorder ; Single ; crystal X ; ray ; Raman spectroscopy ; Photoluminescence spectroscopy
- 刊名：Physics and Chemistry of Minerals
- 出版年：2015
- 出版时间：April 2015
- 年：2015
- 卷：42
- 期：4
- 页码：251-260
- 全文大小：1,234 KB
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17. Lenaz, D, Lughi, V (2013) Raman study of MgCr2O4–Fe2+Cr2O4 and MgCr2O4–MgFe2 3+O4 synthetic series: the effects of Fe2+ and Fe3+ on Raman shifts. Phys Chem Minerals 40: pp. 491-498 CrossRef
18. Lenaz, D, Skogby, H, Princivalle, F, H?lenius, U (2004) Structural changes and valence states in the MgCr2O4–FeCr2O4 solid solution series. Phys Chem Minerals 31: pp. 633-642 CrossRef
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- 刊物主题：Earth sciences
Mineralogy
Crystallography
Geochemistry
Mineral Resources - 出版者：Springer Berlin / Heidelberg
- ISSN：1432-2021
文摘
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)??. 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)?? and T–O distances decrease to 1.9130(8)??. The crossover temperature, at which T–O and M–O distances become equal (i.e., 1.927??), 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? 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.