阿富汗浅绿色电气石宝石学及谱学特征

doi:10.3964/j.issn.1000-0593(2023)07-2195-07

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Abstract In this paper, the coloration mechanism of green tourmaline was determined by conducting a series of tests on the light green tourmaline mined from Afghanistan, including routine gemological tests, XRD analysis, FTIR Analysis, UV-Vis spectral analysis, XPS analysis, and electron probe microanalysis. The results showed that the tourmaline has a relative density of 3.04, an ordinary refractive index (No) of 1.639, an extraordinary refractive index (Ne) of 1.620, and weak pleochroism. XRD analysis indicated that it is lithium tourmaline. FTIR spectra showed absorption peaks at 456, 500, 605, 645, 715, 780, 980, 1 030, 1 110, 1 290, 1 350, 3 460, 3 580 and 3 640 cm^-1, etc. Among them, the peaks at 605, 645, 715, 780, and 1 110 cm^-1 were caused by the symmetric and asymmetric stretching vibrations of Si—O—Si; the peaks at 980 and 1 030 cm^-1 were caused by the symmetric and asymmetric stretching vibrations of O—Si—O; the peak at 500 cm^-1 was caused by the bending vibrations of Si—O; the peak at 1 290 cm^-1 was caused by the stretching vibrations of [BO₃]; the peak at 1 350 cm^-1 was caused by the bending vibrations of OH; the peaks at 3 460 and 3 580 cm^-1 were caused by O3H vibrations; O1H vibrations caused the peak at 3 640 cm^-1; and the strong peak at 456 cm^-1 was caused by [AlO₆] vibrations. The differences between the absorption peaks at 605 and 645 cm^-1 in the measured FTIR spectrum and those in the standard FTIR spectrum may indicate that the presence of color-producing ions has some impact on [Si₆O₁₈] vibrations. XRD and FTIR analyses revealed the underlying crystal structure resulting in the light green color. In the visible light range, the absorption peaks of the tourmaline in E⫽c and E⊥c directions are roughly at the same position, and only differ slightly in absorption intensity, which results in weak pleochroism of the tourmaline. Absorption was found at both 718 nm in the red region and 420 nm in the blue-violet region, whereas good transmission was detected in the yellow-green region, which produced the tourmaline’s unique color of bright light green. UV-Vis spectral analysis revealed the color structure of the light green. XPS analysis showed that the tourmaline mainly contains Li, Na, Al, Si, O, F, B, and other elements. It also contains traces of transition metal ions such as Fe²⁺, Fe³⁺, Mn²⁺, and Ni²⁺, of which Fe²⁺, Fe³⁺, and Ni²⁺ occupy the Y site and Mn²⁺ occupies the Z site. XPS analysis revealed the types, valence states, occupancy, and other chemical states of the transition metal ions that produce the light green color. In combination with the results of electron probe microanalysis, the crystal chemical formula of the sample can be estimated as ^X(Na_0.612Ca_0.063K_0.008)^Y(Li_0.989Fe²⁺_0.070Fe³⁺_0.117Al_1.824)^Z(Mn²⁺_0.035Al_5.762Si_0.203)[Si_6.000O₁₈][BO₃]₃^V(OH_2.134O_0.866)^W(OH_0.542F_0.458). Electron probe microanalysis revealed the chemical composition of the crystal responsible for the production of light green. Comprehensive analysis of the UV-Vis spectrum and chemical components of the sample as well as the chemical states of the transition metal ions contained in the sample suggested that the absorption at 718 nm may be caused by charge transfer from Fe²⁺ to Fe³⁺, where the absorption at 420 nm may be caused by the d—d electron transition of Ni²⁺. These study results may provide a reliable basis for color change optimization based on the chemical state of color-producing ions and the place of origin identification based on crystal chemistry and spectroscopic characteristics.

Key words：Tourmaline; Afghanistan; Crystal chemistry; Transition metal; Coloring mechanism

Received: 2021-11-29 Accepted: 2022-07-08

ZTFLH:	P742
	P571

Cite this article:

LI Ming,HONG Han-lie. Gemological and Spectrographic Characteristics of Light-Green Tourmaline of Afghanistan[J]. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2023, 43(07): 2195-2201.

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2023)07-2195-07 OR https://www.gpxygpfx.com/EN/Y2023/V43/I07/2195

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