Spectral Characteristics and Color Mechanism of Heat-Treated Gem-Quality Yellow Sphene
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-1 are 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).
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