Spectroscopic Characteristics and Color Genesis of Yellowish-Green
Montebrasite
ZHANG Juan1, LI Ke-xin2, QIN Dong-mei1, BAO De-qing1, 2, WANG Chao-wen2*
1. Arts and Media Institute, Wuhan University of Engineering Science, Wuhan 430200, China
2. Gemmological Institute, China University of Geosciences (Wuhan), Wuhan 430074, China
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 PO3-4 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 PO3-4. The absorptions at ~832 and 3 386 cm-1 were linked to the bending vibration of OH (δOH) and stretching vibration of OH (νOH), 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 PO3-4 (ν3). The Raman peaks below 600 cm-1 were more complex, relating to the asymmetric stretching vibration of PO3-4 (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 nγ, 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 O2-—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
张 娟，李可心，秦冬梅，包德清，王朝文. 黄绿色羟基磷铝锂石的谱学特征及其颜色成因[J]. 光谱学与光谱分析, 2024, 44(03): 777-783.
ZHANG Juan, LI Ke-xin, QIN Dong-mei, BAO De-qing, WANG Chao-wen. Spectroscopic Characteristics and Color Genesis of Yellowish-Green
Montebrasite. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2024, 44(03): 777-783.
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