• Spectroscopy and Spectral Analysis
  • Vol. 44, Issue 9, 2545 (2024)
WANG Pei-lian1, YUE Su-wei2、*, and LI Jia-yan1
Author Affiliations
  • 1[in Chinese]
  • 2[in Chinese]
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    DOI: 10.3964/j.issn.1000-0593(2024)09-2545-06 Cite this Article
    WANG Pei-lian, YUE Su-wei, LI Jia-yan. Spectral Characteristics and Color Mechanism of Heat-Treated Gem-Quality Yellow Sphene[J]. Spectroscopy and Spectral Analysis, 2024, 44(9): 2545 Copy Citation Text show less

    Abstract

    Sphene is known as titanite, which is a transparent neosilicate mineral that can be used as a gem. Other elements can substitute the cations Ca2+, Ti4+, and Si4+ in sphene. Yellow sphene was selected for heat treatment to explore the heat treatment process of sphene. The results showed that the yellow sphene samples were heated in an oxidizing atmosphere at 700 ℃ and turned brown-red above, whereas, the reducing atmosphere heated again to turn yellow again. The composition analyzed by the electron microprobe analysis (EMPA) shows that the samples are consistent with the standard sphene composition, and also contain a small amount of FeO (average 0.601 wt%) and Al2O3 (average 0.210 wt%). The mid-infrared reflectance spectra 1 600~400 cm-1are consistent with standard titanite, showing absorptions attributed to O—Si—O and Si—O vibrations. In the mid-infrared transmission spectra 4 000~2 000 cm-1, the absorption is caused by O—H stretching vibration and accompanied by 3 450 cm-1 centered absorptions broadband. In the near-infrared transmission spectra 10 000~4 000 cm-1, the absorption is caused by O—H stretching vibration. The UV-visible absorption spectrum illustrates that the yellow color of the untreated titanite sample is due to the substitution of Fe2+as an isomorphic counterpart for Ti4+ in the octahedral site, leading to the charge transfer forms of Fe2+ to Ti4+ (IVCT), generating an absorption band at 420 nm. The charge transfer between O2-→Fe2+ and O2-→Fe3+ causes the strong absorption towards the ultraviolet at 450 nm. The brownish-red color after oxidation treatment is attributed to the partial conversion of Fe2+ to Fe3+, resulting in the overlapping of the strong absorption edge at 540 and 450 nm towards the ultraviolet respectively, caused by the charge transfer forms of Fe2+ to Fe3+ (IVCT).
    WANG Pei-lian, YUE Su-wei, LI Jia-yan. Spectral Characteristics and Color Mechanism of Heat-Treated Gem-Quality Yellow Sphene[J]. Spectroscopy and Spectral Analysis, 2024, 44(9): 2545
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