黄绿色羟基磷铝锂石的谱学特征及其颜色成因

doi:10.3964/j.issn.1000-0593(2024)03-0777-07

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Abstract The amblygonite-montebrasite series are a kind of rare mineral gemstone which shows a complete solid solution containing end members amblygonite (LiAlPO₄F) and montebrasite (LiAlPO₄OH). However, little research has been paid attention to amblygonite-montebrasite in China, especially on the rapid and non-destructive estimation of F contents to determine the variety and the origin of color. In this study, conventional gemology, Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), Infrared spectroscopy (IR), Raman spectroscopy (Raman), UV-VIS absorption spectroscopy (UV-VIS) were employed to test and analyze the gemstone-grade yellowish-green amblygonite-montebrasite series collected in the market. The results of conventional gemological analyses showed that the high refractive index and low relative density of the amblygonite-montebrasite series indicated that the studied samples belong to the endmember of montebrasite. The infrared spectra of the studied samples in the fingerprint region of 400~1 500 cm^-1 showed an absorption band of 430 cm^-1, belonging to the bending vibration of the Li—O bond. The absorptions of 494, 546, 604 and 664 cm^-1 were caused by the joint actions of the bending vibration (ν₄) of PO^3-₄ and the stretching vibration of the AlO₆ bond. The absorptions at ~1 032, ~1 090 and ~1 211 cm^-1 were owing to the stretching vibration (ν₃) of PO^3-₄. The absorptions at ~832 and 3 386 cm^-1 were linked to the bending vibration of OH (δ_OH) and stretching vibration of OH (ν_OH), respectively. Raman spectra of the samples showed typical Raman peaks at 305, 430, 489, 648, 806, 893, 1 013, 1 058, 1 107, 1 193, 3 367 cm^-1, respectively. The wide peak at ~3 367 cm^-1 was the typical OH stretching vibration, and a group of peaks between 1 000 and 1 200 cm^-1 were the asymmetric stretching vibration of PO^3-₄ (ν₃). The Raman peaks below 600 cm^-1 were more complex, relating to the asymmetric stretching vibration of PO^3-₄ (ν₄) and the stretching vibration of the Al—O bond. Integrated estimations of F contents by using high-value refractive index, infrared and Raman spectral index indicated that the high refractive index n_γ, the difference between the peak values of 3 367 cm^-1 and of 1 058 cm^-1 in Raman spectrum, and the FWMH index of the Raman peak near 3 367 cm^-1 were good indicators of F contents in the series of amblygonite-montebrasite. It can quickly and indirectly semi-quantify the F contents of faceted amblygonite-montebrasite gemstones. The results of LA-ICP-MS showed that the main element compositions of the studied samples were P₂O₅, Al₂O₃ and Li₂O, with a trace amount of Fe, consistent with the chemical composition of amblygonite-montebrasite. The Fe replaced the Al in the octahedral structure of amblygonite-montebrasite. The charge transfer between O^2-—Fe³⁺, the intervalence charge-transfer transition between Fe³⁺—Fe²⁺, and spin forbidden transition of Fe³⁺ d—d orbit caused the absorptions of the blue-violet and the orange-red regions were consistent with the results of UV-VIS spectroscopy with observed absorptions in the blue-violet region (300～420 nm) and the yellow-orange region (near 590 nm), which may be the main reason for the color genesis of amblygonite-montebrasite showing yellowish-green tone.

Key words：Montebrasite; LA-ICP-MS; UV-Vis spectrum; Raman spectrum; Color origin

Received: 2022-08-14 Accepted: 2022-12-01

ZTFLH:

P575.4

Corresponding Authors: WANG Chao-wen E-mail: c.w.wang@cug.edu.cn

Cite this article:

ZHANG Juan,LI Ke-xin,QIN Dong-mei, et al. Spectroscopic Characteristics and Color Genesis of Yellowish-Green Montebrasite[J]. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2024, 44(03): 777-783.

URL:

https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2024)03-0777-07 OR https://www.gpxygpfx.com/EN/Y2024/V44/I03/777